Grade 9: Mathematics Unit 3 Variation

Mathematics
Learner’s Material
9
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Department of Education
Republic of the Philippines
Module 3:
Variations

Math atics G a 9
Learner’s Material
First Edition, 2014
ISBN: 978-971-9601-71-5
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Table of Contents
UN
Module 3. Variations........................................................................................................... 187
Module Map .................................................................................................................................. 189
Pre-Assessment............................................................................................................................ 190
Learning Goals and Targets...................................................................................................... 192
Lesson 1. Direct Variation.......................................................................................................... 194
Lesson 2. Inverse Variation....................................................................................................... 206
Lesson 3. Joint Variation............................................................................................................ 215
Lesson 4. Combined Variation................................................................................................. 220
Glossary of Terms......................................................................................................................... 223
References and Websites Links Used in this Module...................................................... 224

MODULE 3
Variations
I. INTRODUCTION AND FOCUS QUESTIONS
Do you know that an increasing demand for paper contributes to the destruction of trees from
which papers are made?
If waste papers were recycled regularly, it would help prevent the cutting down of trees, global
warming and other adverse effects that would destroy the environment. Paper recycling does
not only save the earth but also contributes to the economy of the country and to the increase
in income of some individuals.
This is one situation where questions such as “Will a decrease in production of paper con-
tribute to the decrease in the number of trees being cut?” can be answered using the concepts
of variations.
There are several relationships of quantities that you will encounter in this situation. You
will learn how a change in one quantity could correspond to a predictable change in the other.
187

In this module you will find out the relation between quantities. Remember to search for
the answer to the following question(s):
• How can I make use of the representations and descriptions of a given set of data?
• What are the beneficial and adverse effects of studying variation which can help solve
problems in real life?
You will examine these questions when you take the following lessons.
II. LESSONS and COVERAGE
Lesson 1 – Direct Variation
Lesson 2 – Inverse Variation
Lesson 3 – Joint Variation
Lesson 4 – Combined Variation
Objectives
In these lessons, you will learn the following:
Lesson 1
• illustrate situations that involve direct variation
• translate into variation statement a relationship involving direct variation
between two quantities given by a table of values, a mathematical equation, and
a graph, and vice versa.
• solve problems involving direct variations.
Lesson 2
• illustrate situations that involve inverse variation
• translate into variation statement a relationship involving inverse variation
between two quantities given by a table of values, a mathematical equation, and
a graph, and vice versa.
• solve problems involving inverse variations.
Lesson 3
• illustrate situations that involve joint variation
• translate into variation statement a relationship involving joint variation
between two quantities given by a mathematical equation, and vice versa.
• solve problems involving joint variations.
Lesson 4
• illustrate situations that involve combined variation
• translate into variation statement a relationship involving combined variation
between two quantities given by a mathematical equation, and vice versa.
• solve problems involving combined variations.
188

Module Map
Here is a simple map of the above lessons your students will cover:
To do well in this module, you will need to remember and do the following:
1. Study each part of the module carefully.
2. Take note of all the formulas given in each lesson.
3. Have your own scientific calculator. Make sure you are familiar with the keys and func-
tions of your calculator.
189
Variations
Combined
Variations
Joint
Variations
Inverse
Variations
Direct
Variations
Table of
Values
Mathematical
Equations
Graphs
Applications
Mathematical
Equations
Applications

III. Pre-assessment
Part I
Let’s find out how much you already know about this topic. On a separate sheet, write only the
letter of the choice that you think best answers the question. Please answer all items. During
the checking, take note of the items that you were not able to answer correctly and look for the
right answers as you go through this module.
1. The cost c varies directly as the number n of pencils is written as
a. c = kn b. k = cn c. n =
k
c
d. c =
k
n
2. The speed r of a moving object is inversely proportional to the time t travelled is written as
a. r = kt b. r =
k
t
c. t = kr d.
r
k
= r
3. Which is an example of a direct variation?
a. xy = 10 b. y =
2
x
c. y = 5x d.
2
y
= x
4. A car travels a distance of d km in t hours. The formula that relates d to t is d = kt. What
kind of variation is it?
a. direct b. inverse c. joint d. combined
5. y varies directly as x and y = 32 when x = 4. Find the constant of variation.
a. 8 b. 36 c. 28 d. 128
6. Which of the following describes an inverse variation?
a.
x 2 3 4 5
y 5
10
3
5
2
2
c. x 40 30 20 10
y 8 6 4 2
b.
x 1 2 3 4
y 5 10 15 20
d.
x 4 8 10 12
y 2 4 5 6
7. What happens to T when h is doubled in the equation T = 4h?
a. T is halved c. T is doubled
b. T is tripled d. T becomes zero
8. If y varies directly as x and y = 12 when x = 4, find y when x = 12.
a. 3 b. 4 c. 36 d. 48
190

9. What mathematical statement describes the graph below?
a. lw = 36 b.
l
w
= 36 c.
l
36
= w d.
w
36
= l
10. If y varies inversely as x and y =
1
3
when x = 8, find y when x = -4.
a.
–2
3
b.
2
3
c.
–32
3
d.
32
3
11. What happens to y when x is tripled in the relation y =
k
x
?
a. y is tripled. c. y is halved.
b. y is doubled. d. y is divided by 3.
12. w varies directly as the square of x and inversely as p and q. If w = 12 when x = 4, p = 2 and
q = 20, find w when x = 3, p = 8 and q = 5.
a. 10 b. 9 c.
27
4
d. 5
13. If 3 men can do a portion of a job in 8 days, how many men can do the same job in 6 days?
a. 7 b. 6 c. 5 d. 4
14. If y varies inversely as x, and y =
1
5
when x = 9, find y when x = -3.
a. 5 b.
1
3
c.
3
5
d. –
3
5

15. Mackee’s income varies directly as the number of days that she works. If she earns
Php 8,000.00 in 20 days, how much will she earn if she worked 3 times as long?
a. Php 26,000 b. Php 24,000 c. Php 20,000 d. Php 16,000
191

16. If s varies directly as t and inversely as v, then which of the following equations describes the
relation among the three variables s, t, and v?
a. s =
k
tv
b. s =
kv
t
c.
1
s
=
kt
v
d. s =
kt
v
17. If (x – 4) varies inversely as (y + 3) and x = 8 when y = 2, find x when y = -1.
a. 20 b. 18 c. 16 d. 14
18. The amount of gasoline used by a car varies jointly as the distance travelled and the square
root of the speed. Suppose a car used 25 liters on a 100 km trip at 100 kph, about how many
liters will it use on a 1000-km trip at 64 kph?
a. 100 L b. 200 L c. 300 L d. 400 L
19. If y varies directly as the square of x, how is y changed if x is increased by 20%?
a. 44% decrease in y c. 0.44% decrease in y
b. 44% increase in y d. 0.44% increase in y
20. If h varies jointly as j2
and i and inversely as g, and h = 50 when j = 2, i = 5, and g =
1
2
, find
h when j = 4, i = 10, and g =
1
4
.
a. 25 b. 100 c. 800 d. 805
IV. Learning Goals and targets
After going through this module, you should be able to demonstrate understanding of key
concepts of variations, to formulate real-life problems involving these concepts, and to solve
these using a variety of strategies. Furthermore, you should be able to investigate mathematical
relationships in various situations involving variations.
192

Let us begin with exploratory activities that will introduce you to the basic concepts of variation
and how these concepts are applied in real life.
➤ Activity 1: Before Lesson Response
Read the phrases found at the right column in the table below. If the phrase is a direct variation, place
a letter D in the Before Lesson Response column, if it is an inverse variation, place a letter I. If the
relationship is neither a direct nor inverse variation, mark it N.
Before Lesson Response Phrase
1. The number of hours to finish a job to the number of
men working
2. The amount of water to the space that water did not
occupy in a particular container
3. The number of persons sharing a pie to the size of the
slices of the pie
4. The area of the wall to the amount of paint used to
cover it
5. The time spent in walking to the rate at which a person
walks
6. The time a teacher spends checking papers to the
number of students
7. The cost of life insurance to the age of the insured
person
8. The age of a used car to its resale value
9. The amount of money raised in a concert to the
number of tickets sold
10. The distance an airplane flies to the time travelling
193

Direct Variation
What to Know
Let’s start the module by doing activities that will reveal your background knowledge on
direct variations. These are practical situations that you also encounter in real life.
➤ Activity 2: What’s the Back Story?
Read and analyze the situation below and answer the questions that follow.
Helen and Joana walk a distance of one kilometer in going to the school where they teach.
At a constant rate, it takes them 20 minutes to reach school in time for their first class.
1
One particular morning, the two became so engrossed in discussing an incident inside the
school during the previous day that they did not notice that the pace at which they were walking
slowed down.
Questions:
a. How will they be able to catch up for the lost number of minutes? Cite solutions.
b. How are the quantities like rate, time, and distance considered in travelling?
c. Does the change in one quantity affect a change in the other? Explain.
194

➤ Activity 3: Let’s Recycle!
A local government organization launches a recycling campaign of waste materials to schools in
order to raise students’ awareness of environmental protection and the effects of climate change.
Every kilogram of waste materials earns points that can be exchanged for school supplies and
grocery items. Paper, which is the number one waste collected, earns 5 points for every kilo.
The table below shows the points earned by a Grade 8 class for every number of kilograms of waste
paper collected.
Number of kilograms (n) 1 2 3 4 5 6
Points (P) 5 10 15 20 25 30
Questions:
1. What happens to the number of points when the number of kilograms of paper is doubled?
tripled?
2. How many kilograms of paper will the Grade 8 class have to gather in order to raise
500 points? Write a mathematical statement that will relate the two quantities involved.
3. In what way are you able to help clean the environment by collecting these waste papers?
4. What items can be made out of these papers?
195

➤ Activity 4: How Steep Is Enough?
Using his bicycle, Jericho travels a distance of 10 kilometers per hour on a steep road. The
table shows the distance he has travelled at a particular length of time.
Time (hr) 1 2 3 4 5
Distance (km) 10 20 30 40 50
Questions:
1. What happens to the distance as the length of time increases?
2. Using this pattern, how many kilometers would he have travelled in 8
1
2
hours?
3. How will you be able to find the distance (without the aid of the table)? Write a mathematical
statement to represent the relation.
4. What mathematical operation did you apply in this case? Is there a constant number involved?
Explain the process that you have discovered.
How did you find the four activities? I am sure you did not find any difficulty in answering
the questions.
The next activities will help you fully understand the concepts behind these activities.
196

What to Process
There is direct variation whenever a situation produces pairs of numbers in which their
ratio is constant.
The statements:
“y varies directly as x”
“y is directly proportional to x” and
“y is proportional to x”
may be translated mathematically as y = kx, where k is the constant of variation.
For two quantities, x and y, an increase in x causes an increase in y as well. Similarly, a
decrease in x causes a decrease in y.
➤ Activity 5: Watch This!
If the distance d varies directly as the time t, then the relationship can be translated into a math-
ematical statement as d = kt, where k is the constant of variation.
Likewise, if the distance d varies directly as the rate r, then the mathematical equation
describing the relation is d = kr.
In Activity 4, the variation statement that is involved between the two quantities is d = 10t.
In this case, the constant of variation is k = 10.
Using a convenient scale, the graph of the relation d = 10t is a line.
The graph above describes a direct variation of the form y = kx.
Which of the equations is of the form y = kx and shows a direct relationship?
1. y = 2x + 3 4. y = x2
– 4
2. y = 3x 5. y = 4x2
3. y = 5x
197

Your skill in recognizing patterns and knowledge in formulating equations helped you answer
the questions in the previous activities. For a more detailed solution of problems involving direct
variation, let us see how this is done.
Examples:
1. If y varies directly as x and y = 24 when x = 6, find the variation constant and the equation
of variation.
Solution:
a. Express the statement “y varies directly as x” as y = kx.
b. Solve for k by substituting the given values in the equation.

y = kx
24 = 6k
k =
24
6
k = 4
Therefore, the constant of variation is 4.
c. Form the equation of the variation by substituting 4 in the statement, y = kx.
y = 4x
2. The table below shows that the distance d varies directly as the time t. Find the constant of
variation and the equation which describes the relation.
Time (hr) 1 2 3 4 5
Distance (km) 10 20 30 40 50
Solution:
Since the distance d varies directly as the time t, then
d = kt.
Using one of the pairs of values, (2, 20), from the table, substitute the values of d and t
in d = kt and solve for k.
d = kt
20 = 2k
k =
20
2
k = 10
Therefore, the constant of variation is 10.
Form the mathematical equation of the variation by substituting 10 in the statement
d = kt.
d = 10t
198

We can see that the constant of variation can be solved if one pair of values of x and y is
known. From the resulting equation, other pairs having the same relationship can be obtained.
Let us study the next example.
3. If x varies directly as y and x = 35 when y = 7, what is the value of y when x = 25?
Solution 1.
Since x varies directly as y, then the equation of variation is in the form x = ky.
Substitute the given values of y and x to solve for k in the equation.

35 = k 7( )
k =
35
7
k = 5
Hence, the equation of variation is x = 5y.
Solving for y when x = 25,

25 = 5y
y =
25
5
y = 5
Hence, y = 5.
Solution 2.
Since
x
y
is a constant, then we can write k =
x
y
. From here, we can establish a proportion
such that
x1
y1
=
x2
y2
where x1
= 35, y1
= 7 and x2
= 25.
Substituting the values, we get

35
7
=
25
y2
5 =
25
y2
y2
=
25
5
y2
= 5
Therefore, y = 5 when x = 25.
Now, let us test what you have learned from the discussions.
199

➤ Activity 6: It’s Your Turn!
A. Write an equation for the following statements:
1. The fare F of a passenger varies directly as the distance d of his destination.
2. The cost C of fish varies directly as its weight w in kilograms.
3. An employee’s salary S varies directly as the number of days d he has worked.
4. The area A of a square varies directly as the square of its side s.
5. The distance D travelled by a car varies directly as its speed s.
6. The length L of a person’s shadow at a given time varies directly as the height h of the
person.
7. The cost of electricity C varies directly as the number of kilowatt-hour consumption l.
8. The volume V of a cylinder varies directly as its height h.
9. The weight W of an object is directly proportional to its mass m.
10. The area A of a triangle is proportional to its height h.
B. Determine if the tables and graphs below express a direct variation between the variables.
If they do, find the constant of variation and an equation that defines the relation.
1. x 1 2 3 4
y -3 -6 -9 -12
2. x 7 14 -21 -28
y 3 6 -9 -12
3. x -15 10 -20 25
y -3 2 -4 5
4. x 2 3 4 5
y 1 2 3 4
5. x 16 20 24 28 32
y 12 15 18 21 24
6. x 6 8 10 12
y 7 9 11 13
200

7.
8.
9. 30
Y
X
25
20
15
10
5
0
0 1 2 3 4 5 6
10.
201

C. Write an equation where y varies directly as x.
1. y = 28 when x = 7 6. y = 63 when x = 81
2. y = 30 when x = 8 7. y = 200 when x = 300
3. y = 0.7 when x = 0.4 8. y = 1 and x = 2
4. y = 0.8 when x = 0.5 9. y = 48 and x = 6
5. y = 400 when x = 25 10. y = 10 and x = 24
D. In each of the following, y varies directly as x. Find the values as indicated.
1. If y = 12 when x = 4, find y when x = 12
2. If y = -18 when x = 9, find y when x = 7
3. If y = -3 when x = -4, find x when y = 2
4. If y = 3 when x = 10, find x when y = 1.2
5. If y = 2.5 when x = .25, find y when x = .75
What to Reflect and UNDERSTAND
Having developed your knowledge about the concepts of direct variation, your goal now is
to take a closer look at some aspects of the lesson. This requires you to apply these concepts
in solving the problems that follow
➤ Activity 7: Cans Anyone?
1. Tin cans of beverages are collected for recycling purposes in many places in the Philippines.
202

Junk shops pay Php 15.00 for every kilo of tin cans bought from collectors. In the following
table, c is the cost in peso and n is the number of kilos of tin cans:
n 1 2 3 4 5 6
c 15 30 45 60 75 90
a. Write a mathematical statement that relates the two quantities n and c.
b. What is the constant of variation? Formulate the mathematical equation.
c. Observe the values of c and n in the table. What happens to the cost c when the num-
ber n of kilos of paper is doubled? Tripled?
d. Graph the relation.
e. How much would 20 kilos of tin cans cost if at the end of month, the cost for every
kilo of tin cans will increase by 5 pesos?
f. What items can be made out of these tin cans?
2. The circumference of a circle varies directly as its diameter. If the circumference of a circle
having a diameter of 7 cm is 7 π cm, what is the circumference of the circle whose diameter
is 10 cm? 15 cm? 18 cm? 20 cm?
a. Write a mathematical statement that relates the two quantities involved in the problem.
b. What is the constant of variation? Formulate the mathematical equation.
c. Construct a table of values from the relation.
3. The service fee f of a physical therapist varies directly as the number of hours n of service
rendered. A physical therapist charges Php 2,100 for 3 hours service to patients in a home
care. How much would he be paid for 6
1
2
hours of service?
4. The amount of paint p needed to paint the walls of a room varies directly as the area A of the
wall. If 2 gallons of paint is needed to paint a 40 sq meter wall, how many gallons of paint
are needed to paint a wall with an area of 100 sq meters?
5. A teacher charges Php 500 per hour of tutorial service to a second year high school student.
If she spends 3 hours tutoring per day, how much would she receive in 20 days?
6. A mailman can sort out 738 letters in 6 hours. If the number of sorted letters varies directly
as the number of working hours, how many letters can be sorted out in 9 hours?
7. Jessie uses 20 liters of gasoline to travel 200 kilometers, how many liters of gasoline will he
use on a trip of 700 kilometers?
8. Every 3 months, a man deposits in his bank account a savings of Php 5,000. In how many
years would he have saved Php 250,000?
9. The pressure p at the bottom of a swimming pool varies directly as the depth d of the water.
If the pressure is 125 Pascal when the water is 2 meters deep, find the pressure when it is
4.5 meters deep.
10. The shadow of an object varies directly as its height h. A man 1.8 m tall casts a shadow 4.32 m
long. If at the same time a flagpole casts a shadow 12.8 m long, how high is the flagpole?
203

What to TRANSFER
Your goal in this section is to apply what you have learned to real-life situations. This shall be
one of your group’s outputs for the second quarter. A practical task shall be given to your group
where each of you will demonstrate your understanding with clarity and accuracy, and further
supported through refined mathematical justifications along with your project’s stability and
creativity.Your work shall be graded in accordance to a rubric prepared for this task.
➤ Activity 8: GRASPS
Create a scenario of the task in paragraph form incorporating GRASPS: Goal, Role, Audience,
Situation, Product/Performance, Standards. A sample has been provided for you.
Sample of GRASP
Goal or task related to understanding: Your goal is to provide jobs for skilled people in the
community.
Role: Project Manager
Audience: Barangay Officials
Situation or Context of Scenario: As an aspiring businessman, you have been commissioned
by a client to supply them with 10 000 pieces of hanging decors with a Christmas motif.
Given two months, your problem is to find skilled persons to help you meet the deadline.
You have thought of hiring people from your community. If the profit is to be considered,
the task is to find the number of workers that would be hired to generate maximum profit.
Product(s) or Performances for Assessment: Prepare and discuss to the Barangay Officials
the scheme with the appropriate computation of the workers’ wage per piece, the profit
that would be earned by the Barangay from the project. The scheme should be presented
using tables showing wages generated by each at a certain rate. The proposed scheme should
include the amount generated by each after a certain period. Problems/issues associated to
the project should be discussed. Show the necessary plan of action that would be taken to
go about the problems/issues.
Standards for Assessment: Clarity, Accuracy, and Justification
204

RUBRICS FOR PLAN OF ACTION
Criteria 4 3 2 1 Rating
Clarity
The
presentation
is very clear,
precise, and
coherent. It
included
concepts
related
to direct
variation.
The
presentation
is clear,
precise, and
coherent. It
included
concepts
related
to direct
variation.
The
presentation
is vague but
it included
concepts
related
to direct
variation.
The
presentation
is vague
and did
not include
concepts
related
to direct
variation.
Accuracy
The
computations
are accurate
and show
the wise use
of the key
concepts
of direct
variation.
The
computations
are accurate
and show the
use of the
key concepts
of direct
variation.
The
computations
are accurate
and show
some use
of the key
concepts
of direct
variation.
The
computations
are erroneous
and do not
show the
use of the
key concepts
of direct
variation.
Justification
The purpose
is well
justified and
shows the
maximum
and beneficial
profit that
will be gained
from the
project.
The purpose
is well
justified and
shows the
maximum
profit that
will be gained
from the
project.
The purpose
is justified
and shows
minimal
profit that
will be gained
from the
project.
The purpose
is not
justifiable
and shows
no profit that
will be gained
from the
project.
Summary/Synthesis/Generalization:
➤ Activity 9: Wrap It Up!
On a sheet of paper, summarize what you have learned from this lesson. Provide real-life
examples. Illustrate using tables, graphs, and mathematical equations showing the relation
of quantities.
205

Inverse Variation
What to Know
The activities on direct variation show you the behavior of the quantities involved. In one
of the activities, an increase in the time travelled by a car causes an increase in the distance
travelled. How will an increase in speed affect the time in travelling? Let us find out in the
next activity.
➤ Activity 10:
1. Anna lives 40 km away from the office of ABC Corporation where she works. Driving a car,
the time it takes her to reach work depends on her average speed. Some possible speeds and
the length of time it takes her are as follows:
Time in hours 1
4
5
2
3
4
7
1
2
Speed in kph 40 50 60 70 80
To see clearly the relation of the two quantities, the graph of the relation is shown below.
Questions:
a. How do the speed and time of travel affect each other?
b. Write a mathematical statement to represent the relation.
c. Is there a constant number involved? Explain the process that you have used in finding out.
2
1 2 3 4 11 1
5 5 5 5 5
206

The situation in the problem shows “an increase in speed produces a decrease in time in
travelling.” The situation produces pairs of numbers, whose product is constant. Here, the
time t varies inversely as the speed s such that
st = 40 (a constant)
In this situation,“the speed s is inversely proportional to the time t,”and is written as s =
k
t
,where
k is the proportionality constant or constant of variation. Hence, the equation represented
in the table and graph is s =
40
t
; where k = 40.
2. Jean and Jericho who are playing in the school grounds decided to sit on a seesaw.
Jericho, who is heavier, tends to raise Jean on the other end of the seesaw. They tried to
position themselves in order to balance the weight of each other.
Questions:
a. What have you noticed when the kids move closer to or farther from the center?
b. Who among the kids will have to move closer to the center in order to balance the seesaw?
c. How do the weights of the kids relate to the distance from the center?
d. Does the change in one quantity affect a change in the other? Explain.
➤ Activity 11: Observe and Compare
Consider the table of values A and B
Table A Table B

x -2 -1 1 2 3
y -4 -2 2 4 6

x 80 60 40 30 20
y 6 9 12 16 24
207

Compare the two given table of values.
1. What have you observed about the values in both tables?
2. What do you observe about the values of y when x increases/decreases?
3. What happens to the values of y when x is doubled? tripled?
4. How do you compare the two relations?
5. Write the relationship which describes x and y.
6. How can you determine if the relationship is a direct variation or an inverse variation?
What to PROCESS
Let us now discuss the concepts behind the situations that you have encountered. These
situations are examples of inverse variation.
Inverse variation occurs whenever a situation produces pairs of numbers whose product
is constant.
For two quantities x and y, an increase in x causes a decrease in y or vice versa. We can
say that y varies inversely as x or y =
k
x
.
The statement, “y varies inversely to x,” translates to y =
k
x
, where k is the constant of
variation.
Examples:
1. Find the equation and solve for k: y varies inversely as x and y = 6 when x = 18.
Solution:
The relation y varies inversely as x translates to y =
k
x
. Substitute the values to find k:

y =
k
x
6 =
k
18
k = 6( ) 18( )
k = 108
The equation of variation is y =
108
x
208

2. If y varies inversely as x and y = 10 when x = 2, find y when x = 10.
This concerns two pairs of values of x and y which may be solved in two ways.
Solution 1:
First, set the relation, and then find the constant of variation, k.
xy = k
(2)(10) = k
k = 20
The equation of variation is y =
20
x
Next, find y when x = 10 by substituting the value of x in the equation,

y =
20
x
y =
20
10
y = 2
Solution 2:
Since k = xy, then for any pairs x and y, x1
y1
= x2
y2
If we let x1
= 2, y1
= 10, and x2
= 10, find y2
.
By substitution,
x1
y1
= x2
y2
2(10) = 10(y2
)
20 = 10y2
y2
=
20
10
y2
= 2
Hence, y = 2 when x = 10.
➤ Activity 12: It’s Your Turn!
Let us now test your skills in translating statements into mathematical equations and in
finding the constant of variation.
A. Express each of the following statements as a mathematical equation.
1. The number of pizza slices p varies inversely as the number of persons n sharing a
whole pizza.
2. The number of pechay plants n in a row varies inversely as the space s between them.
3. The number of persons n needed to do a job varies inversely as the number of days
d to finish the job.
209

4. The rate r at which a person types a certain manuscript varies inversely as the time
t spent in typing.
5. The cost c per person of renting a private resort varies inversely as the number n of
persons sharing the rent.
6. The length l of a rectangular field varies inversely as its width w.
7. The density d of air varies inversely as the volume v of water in the atmosphere.
8. The acceleration a of a car is inversely proportional to its mass m.
9. The base b of a triangle varies inversely as its altitude h.
10. The mass m of an object varies inversely as the acceleration due to gravity g.
B. Find the constant of variation and write the equation representing the relationship
between the quantities in each of the following:
1. x 1 2 3 4
y 2 1
2
3
1
2
2. x 2 4 6 8
y 36 18 12 9

3. x 3 6 9 12
y 2 1
2
3
1
2
4. x 7 5 3 1
y
5
7
1
5
3
5
5.
6.
210

7. y varies inversely as x and y = 12 when x = 5.
8. y varies inversely as x and y = 18 when x = 3.
9. y varies inversely as x and y = 10 when x =
2
5
.
10. y varies inversely as x and y = 12 when x =
2
3
.
C. Solve for the indicated variable in each of the following.
2. If r varies inversely as s and r = 100 when s = 27, find the value of r when s = 45.
3. If p varies inversely as the square of q and p = 3 when q = 4, find p when q = 16.
4. If y varies inversely as x and y = -2 when x = -8, find x when y = 2.
5. If w varies inversely as y and w = 2 when y = 3, find w when y = 6.
6. If m varies inversely as n and m = 8 when n = 3, find m when n = 12.
7. If y varies inversely as x, and y is 10 when x = 5, find y when x = 7.
8. If a varies inversely as b and a = 12 when b = 8, find a when b = 6.
9. If w varies inversely as x and w is 10 when x = 5 find w when x = 25.
What to REFLECT and UNDERSTAND
Having developed your knowledge of the concepts in this section, your goal now is to apply
these concepts in various situations. As in the previous activities, there will be no guide
questions for you to follow. The situation below is an example of how the problem is solved.
Jamie and Andrea are figuring out a way to balance themselves on a seewaw. Jamie who
weighs 15 kilograms sits 2 meters from the fulcrum. Andrea who weighs 20 kilograms tried
sitting at different distances from the fulcrum in order to balance the weight of Jamie. If you
were Andrea, how far from the fulcrum should you sit?
211

To balance the weight of Jamie, Andrea has to sit at a distance closer to the fulcrum.
The relation shows that the distance d varies inversely as the weight w and can be trans-
formed into a mathematical equation as
d =
k
w
We can now solve for distance from the fulcrum where Andrea has to sit.
Solution:
Let us first solve for k.
k = dw
= 15(2)
k = 30
Solve for the distance of Andrea from the fulcrum.

d =
k
w
=
30
20
d = 1.5m
Hence, Andrea has to sit 1.5 meters from the fulcrum.
➤ Activity 13: Think Deeper!
Solve the following problems.
1 The number of days needed in repairing a house varies inversely as the number of men
working. It takes 15 days for 2 men to repair the house. How many men are needed to com-
plete the job in 6 days?
2. Two college students decided to rent an apartment near the school where they are studying.
The nearest and cheapest apartment costs Php 5,000 a month, which they find too much
for their monthly budget. How many students will they need to share the rent with so that
each will pay only Php 1,250 a month?
3. At 60 kilometers per hour it takes Loida 10 hours to travel from her house to their house in
the province. How long will it take her if she travels at 80 kilometers an hour?
4. The ten administrative staff of a school decided to tour the nearby towns of Laguna on a
particular day. They decided to hire a van for Php 4,000. Two days before the trip, two of
the staff were assigned to attend a seminar and so could no longer join the trip. How much
would each share for the van rental?
5. Joseph is figuring out a way to reach Baguio at the shortest possible time. Using his car, he
can reach Baguio in 6 hours at an average speed of 70 km per hour. How fast should he drive
in order to reach Baguio in 5 hours?
212

Perform Activity 14 and check if your answers are the same as your answers in Activity 1.
This will gauge how well you understood the discussions on direct and inverse variations.
➤ Activity 14: After Lesson Response
Read the phrases found at the right column in the table below. If the phrase is a direct variation,
place a letter D in the After Lesson Response column, if it is an inverse variation, place a letter I.
If the relationship is neither direct nor inverse variation, mark it with a letter N.
After Lesson Response Phrase
1. The number of hours to finish a job to the number of men
working
2. The amount of water to the space that water did not occupy in
a particular container
3. The number of persons sharing a pie to the number of slices of
the pie
4. The area of the wall to the amount of paint used to cover it
5. The time spent in walking to the rate at which a person walks
6. The time a teacher spends checking papers to the number of
students
7. The cost of life insurance to the age of the insured person
8. The age of a used car to its resale value
9. The amount of money raised in a concert to the number of
tickets sold
10. The distance an airplane flies to the time travelling
What to TRANSFER
You are to demonstrate your understanding of inverse variations through culminating
activities that reflect meaningful and relevant problems/situations.
213

➤ Activity 15: Demonstrate Your Understanding!
Create a scenario of the task in paragraph form incorporating GRASP: Goal, Role, Audience,
Situation, Product/Performance, Standards
G: Make a criteria for the best project related to the protection of the environment
R: Student
A: School Officials
S: The school will grant an award to the best project that promotes protection of the
environment.
P: Criteria
S: Justification, Accuracy of Data, Clarity of Presentation
examples. Illustrate using tables, graphs, and mathematical equations showing the relationship
of quantities.
214

Joint Variation
What to Know
The situations that you have studied involved only two quantities. What if the situation
requires the use of more than two quantities?
This lesson deals with another concept of variation, the joint variation.
Some physical relationships, as in area or volume, may involve three or more variables
simultaneously.
You will find out about this type of variation as we go along with the discussions.
What to PROCESS
At this phase, you will be provided with examples that will lead you in solving the problems
you will encounter in this section. The concept of joint variation will help you deal with prob-
lems involving more than two variables or quantities.
The statement“a varies jointly as b and c”means a = kbc, or k =
a
bc
, where k is the constant
of variation.
Examples:
1. Find the equation of variation where a varies jointly as b and c, and a = 36 when b = 3 and
c = 4.
Solution:
a = kbc
36 = k 3( ) 4( )
k =
36
12
k = 3
Therefore, the required equation of variation is a = 3bc
3
215

2. z varies jointly as x and y. If z =16 when x = 4 and y = 6, find the constant of variation and
the equation of the relation.
Solution:
z = kxy
16 = k 4( ) 6( )
k =
16
24
k =
2
3
The equation of variation is z =
2
3
xy
➤ Activity 17: What Is Joint Together?
A. Translate each statement into a mathematical sentence. Use k as the constant of variation.
1. P varies jointly as q and r.
2. V varies jointly as l, w, and h.
3. The area A of a parallelogram varies jointly as the base b and altitude h.
4. The volume of a cylinder V varies jointly as its height h and the square of the radius r.
5. The heat H produced by an electric lamp varies jointly as the resistance R and the square
of the current i.
6. The force F applied to an object varies jointly as the mass m and the acceleration a.
7. The volume V of a pyramid varies jointly as the area of the base B and the altitude h.
8. The area A of a triangle varies jointly as the base b and the altitude h.
9. The appropriate length s of a rectangular beam varies jointly as its width w and its
depth d.
10. The electrical voltage V varies jointly as the current I and the resistance R.
B. Solve for the value of the constant of variation k, then find the missing value.
1. z varies jointly as x and y and z = 60 when x = 5 and y = 6.
a. Find z when x = 7 and y = 6.
b. Find x when z = 72 and y = 4.
c. Find y when z = 80 and x = 4.
2. z varies jointly as x and y. If z = 3 when x = 3 and y = 15, find z when x = 6 and y = 9.
3. z varies jointly as the square root of the product of x and y. If z =3 when x = 3 and
y = 12, find x when z = 6 and y = 64.
4. d varies jointly as h and g. If d = 15 when h = 14 and g = 5, find g when h = 21 and
d = 8.
216

5. q varies jointly as r and s. If q = 2.4, when r = 0.6 and s = 0.8, find q when r = 1.6 and
s = .01.
6. d varies jointly as e and l. If d = 1.2, when e = 0.3 and l = 0.4, find d when e = 0.8 and
l = .005.
7. x varies jointly as w, y, and z. If x = 18, when w = 2, y = 6 and z = 5, find x when w = 5,
y = 12 and z = 3.
8. z varies jointly as x and y. z = 60 when x = 3 and y = 4. Find y when z = 80 and x = 2.
9. The weight W of a cylindrical metal varies jointly as its length l and the square of the
diameter d of its base.
a. If W = 6 kg when l = 6 cm and d = 3 cm, find the equation of variation.
b. Find l when W = 10 kg and d = 2 cm.
c. Find W when d = 6 cm and l = 1.4 cm.
10. The area A of a triangle varies jointly as the base b and the altitude h of the triangle. If
A = 65 cm2
when b = 10 cm and h = 13 cm, find the area of a triangle whose base is 8
cm and whose altitude is 11 cm.
WHAT to REFLECT and UNDERSTAND
Having developed your knowledge on the concepts in the previous activities, your goal now
is to apply these concepts in various real-life situations.
➤ Activity 18: Who Is This Filipino Inventor?
He is a Filipino mathematician who developed a board game called DAMATHS. The board
game applies the moves used in the Filipino board game DAMA to solve problems on the different
concepts in Mathematics. Who is he?
217

To find out, match the letter that corresponds to the answer to the numbered item on your
left. The letters will spell out the name of this Filipino Mathematician.
“z varies jointly as x and y”
1. Translate into variation statement
2. If z = 36 when x = 3 and y = 2, find k.
3. x is 4 when y = 3 and z = 2. What is z if x = 8 and
y = 6.
The area a of a triangle varies jointly as its base b
and height h.
4. Express the relation as a variation statement.
5. If the area is 15 cm2
when the base is 5 cm and
the height is 6 cm, find k.
6. If a = 65 cm2
when b = 10 cm and h = 13 cm,
find a when b = 8 cm and h = 11 cm.
Answer: ___ ___ ___ ___ ___ ___
1 2 3 4 5 6
➤ Activity 19: Think Deeper!
Solve the following:
1. The amount of gasoline used by a car varies jointly as the distance travelled and the square
root of the speed. Suppose a car used 25 liters on a 100 kilometer trip at 100 km/hr. About
how many liters will it use on a 192 kilometer trip at 64 km/hr?
2. The area of triangle varies jointly as the base and the height. A triangle with a base of 8 cm
and a height of 9 cm has an area of 36 square centimeters. Find the area when the base is 10 cm
and the height is 7 cm.
3. The volume of a right circular cylinder varies jointly as the height and the square of the radius.
The volume of a right circular cylinder, with radius 4 centimeters and height 7 centimeters,
is 112 π cm3
. Find the volume of another cylinder with radius 8 centimeters and height 14
centimeters.
4. The mass of a rectangular sheet of wood varies jointly as the length and the width. When
the length is 20 cm and the width is 10 cm, the mass is 200 grams. Find the mass when the
length is 15 cm and the width is 10 cm.
5. The weight of a rectangular block of metal varies jointly as its length, width and thickness.
If the weight of a 13 dm by 8 dm by 6 dm block of aluminum is 18.2 kg, find the weight of
a 16 dm by 10 dm by 4 dm block of aluminum.
E 8
B a =
kb
h
D
1
2
F z =
kx
y
U 6
N a = kbh
A 44 cm2
H z = kxy
218

What to TRANSFER
Journal writing and portfolio of real-life situations/pictures where concepts of joint varia-
tion are applied.
examples. Illustrate situations using variation statements and mathematical equations to
show the relationship of quantities.
219

Combined Variation
What to KNOW
Combined variation is another physical relationship among variables. This is the kind of
variation that involves both the direct and inverse variations.
What to PROCESS
The statement “z varies directly as x and inversely as y” means z =
kx
y
, or k =
zy
x
, where k
is the constant of variation.
This relationship among variables will be well illustrated in the following examples.
Examples:
1. Translating statements into mathematical equations using k as the constant of variation.
a. T varies directly as a and inversely as b.
T =
ka
b

b. Y varies directly as x and inversely as the square of z.
Y =
kx
z2

The following is an example of combined variation where one of the terms is unknown.
2. If z varies directly as x and inversely as y, and z = 9 when x = 6 and y = 2, find z when x = 8
and y = 12.
Solution:
The equation is z =
kx
y
Solve for k by substituting the first set of values of z, x, and y in the equation.

z =
kx
y
9 =
6k
2
k =
9
3
k = 3
4
220

Now, solve for z when x = 8 and y = 12.
Using the equation z =
3x
y
,

z =
3( ) 8( )
12
z = 2
You may use this example to guide you in solving the activities that follow.
➤ Activity 21: DV and IV Combined!
A. Using k as the constant of variation, write the equation of variation for each of the following.
1. W varies jointly as c and the square of a and inversely as b.
2. P varies directly as the square of x and inversely as s.
3. The electrical resistance R of a wire varies directly as its length l and inversely as the
square of its diameter d.
4. The acceleration A of a moving object varies directly as the distance d it travels and
inversely as the square of the time t it travels.
5. The pressure P of a gas varies directly as its temperature t and inversely as its
volume V.
B. Solve the following
1. If r varies directly as s and inversely as the square of u, and r = 2 when s = 18 and u = 2,
find:
a. r when u = 3 and s = 27.
b. s when u = 2 and r = 4.
c. u when r = 1 and s = 36.
2. p varies directly as q and the square of r and inversely as s.
a. Write the equation of the relation.
b. Find k if p = 40 when q = 5, r = 4 and s = 6.
c. Find p when q = 8, r = 6 and s = 9.
d. Find s when p = 10, q = 5 and r = 2.
3. w varies directly as x and y and inversely as v2
and w = 1200 when x = 4, y = 9 and
v = 6. Find w when x = 3, y = 12, and v = 9.
4. Suppose p varies directly as b2
and inversely as s3
. If p =
3
4
when b = 6 and s = 2, find b
when p = 6 and s = 4.
5. If x varies as the square of y and inversely as z and x = 12 when y = 3 and z = 6, find x
when y = 9 and z = 6.
221

What to REFLECT and UNDERSTAND
Having developed your knowledge of the concepts in the previous activities, your goal now
is to apply these concepts in various real-life situations.
➤ Activity 22: How Well Do You Understand?
Solve the following:
1. The current I varies directly as the electromotive force E and inversely as the resistance R. If
in a system a current of 20 amperes flows through a resistance of 20 ohms with an electro-
motive force of 100 volts, find the current that 150 volts will send through the system.
2. The force of attraction, F of a body varies directly as its mass m and inversely as the square
of the distance d from the body. When m = 8 kilograms and d = 5 meters, F = 100 Newtons.
Find F when m = 2 kilograms and d = 15 meters.
3. The electrical resistance of a wire varies directly as its length and inversely as the square of
its diameter. If a wire 30 meters long and 0.75 mm in diameter has a resistance of 25 ohms,
find the length of a wire of the same material whose resistance and diameter are 30 ohms
and 1.25 mm, respectively.
4. The acceleration a of an object varies directly as the force f exerted and inversely as its mass
m. If the constant of variation is 1, find the acceleration in meters/second2
of a 10 kg object
exerting a force of 10 Newtons.
5. The maximum load m of a beam varies directly as the breadth b and the square of the depth
d and inversely as the length l. If a beam 3.6 m long, 1.2 m wide, and 2.4 m deep can safely
bear a load up to 909 kg, find the maximum safe load for a beam of the same material which
is 3 m long, 6 m wide, and 1.8 m deep.
What to TRANSFER
Design a plan how to market a particular product considering the number of units of the
product sold, cost of the product, and the budget for advertising. The number of units sold
varies directly with the advertising budget and inversely as the price of each product.
222

➤ Activity 23: Wrap It Up?
examples. Illustrate using variation statements and mathematical equations showing the
relation of quantities.
Glossary of Terms
Direct Variation
There is direct variation whenever a situation produces pairs of numbers in which their ratio
is constant.
The statements:
“y varies directly as x”
“y is directly proportional to x” and
“y is proportional to x”
are translated mathematically as y = kx, where k is the constant of variation.
For two quantities x and y, an increase in x causes an increase in y as well. Similarly, a decrease
in x causes a decrease in y.
Inverse Variation
Inverse variation occurs whenever a situation produces pairs of numbers whose product is
constant.
For two quantities x and y, an increase in x causes a decrease in y or vice versa. We can say
that y varies inversely as x or y =
k
x
.
Joint Variation
The statement a varies jointly as b and c means a = kbc, or k =
a
bc
, where k is the constant
of variation.
Combined Variation
The statement t varies directly as x and inversely as y means t =
kx
y
, or k =
ty
x
, where k is
the constant of variation.
223

References
DepEd Learning Materials that can be used as learning resources for the lesson on Variation
a. EASE Modules Year II
b. Distance Learning Modules Year II
Dionio, Janet D., Flordelita G. Male, and Sonia E. Javier, Mathematics II, St. Jude Thaddeus
Publications, Philippines, 2011
Barberan, Richard G., et al., Hyper8 Math II, Hyper8 Publications, Philippines 2010
Atela, Margarita E., Portia Y. Dimabuyu, and Sonia E. Javier, Intermediate Algebra, Neo Asia
Publishing Inc., Philippines 2008
224

Grade 9: Mathematics Unit 3 Variation

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Grade 9: Mathematics Unit 3 Variation