Math for Bus. and Eco. Chapter 1

Chapter One

អអអអអអ អអ
អអអអអអ អអ
អអ
អ
Function

Definition
Let D and R be two sets of real numbers. A
function f is a rule that matches each
Chapter 1: Function

number x in D with exactly one and only
one number y or in . D is called the
domain of f and is called the range of f.
The letter x is sometimes referred to as
independent variable and y dependent
variable.

2

Example 1
A real estate broker charges a commission
of 6% on Sales valued up to $300,000. For
sales valued at more than $ 300,000, the
Chapter 1: Function

commission is $ 6,000 plus 4% of the sales
price.
a) Represent the commission earned as
a function R
b) Find R (200,000).
c) Find R (500,000).

3

0.06 x for 0 x 300,000
a) R x
0.04 x 6000 for x 300,000
Chapter 1: Function

b) R 200,000 0.06 200,000 $12,000

c) R 500, 000 0.04 500, 000 6000
$26, 000
4

1.2 Domain of a Function
Chapter 1: Function

The set of values of the independent
variables for which a function can be
evaluated is called the domain of a
function.

5

Example 2
Find the domain of each of the following
functions:
Chapter 1: Function

1
a) f x
x 3

b) g x x 2

6

Example 2 (Continued)
a) The function is defined if
x 3 0
Chapter 1: Function

x 3
Hence, the domain is  3

b) The function is defined if x

7

1.3 Composition of Functions
The composite function g[h(x)] is the
function formed from the two functions g[u]
Chapter 1: Function

and h(x) by substituting h(x) for u in the
formula for g[u].

8

1.3 Composition of Functions
An environmental study of a certain
community suggests that the average daily
Chapter 1: Function

level of carbon monoxide in the air will be
C(p) 0.5p 1 parts per million when the
population is p thousand. It is estimated that
t years from now the population of the
community will be p(t) 10 0.1 t2
thousand.

9

1.3 Composition of
Functions(Cont.)
a) Express the level of carbon monoxide in
the air as a function of time.
Chapter 1: Function

b) When will the carbon monoxide level
reach 6.8 parts per million?

10

a) Level of carbon monoxide as the
function of time
2
C P t C 10 0.1t
Chapter 1: Function

2
0.5 10 0.1 t 1
2
6 0.05 t
b) The time when the carbon monoxide level
reach 6.8 parts per million

11

b) The time when the carbon monoxide
level reach 6.8 parts per million
Chapter 1: Function

2
6 0.05t 6.8
2
0.05t 0.8
2
t 16
t 4
4 years from now the level of carbon
monoxide will be 6.8 parts per million.

12

2 The Graph of a Function
Chapter 1: Function

The graph of a function f consists of all
points (x, y) where x is in the domain of f
and .

13

How to Sketch the Graph of a Function f
by Plotting Points
Chapter 1: Function

 Choose a representative collection of
numbers x from the domain of f and
construct a table of function values for
those numbers.
 Plot the corresponding points
 Connect the plotted points with a smooth
curve.

14

Graph of y ax2 bx c
Chapter 1: Function

o It is a parabola, which is of U shape. It
opens either up if y 0 or down if y 0.
o The peak or valley of a parabola is
called vertex whose x-coordinate is
x b/2a

15

Tips for sketching a parabola
y ax2 bx c
Chapter 1: Function

 Locate the vertex
 Determine whether it opens up or down.
 Find intercepts if any.

16

Example 4
For the equation
oFind the Vertex
Chapter 1: Function

oFind the minimum value for y
oFind the x-intercepts.
oSketch the graph.

17

Example 4
Y
Chapter 1: Function

y=x2-6x+4

X

(3,-5)

18

3 Linear Functions
 The Slope of a Line
The slope of a line is the amount by
Chapter 1: Function

which the y-coordinate of a point on
the line changes when the x
coordinate is increased by 1.

19

The Slope of a Line
The slope of the non-vertical line passing
through the points (x1,y1) and (x2,y2) is
Chapter 1: Function

given by the formula
y y2 y1
Slope
x x2 x1

20

The Slope of a Line

y
Chapter 1: Function

x 2 , y2

y2 y1 y
x1 , y1
x2 x1 x
x

21

Horizontal and Vertical Lines

y
Chapter 1: Function

(0,b) y=b

x

Horizontal line

22

Horizontal and Vertical Lines
y
x=c
Chapter 1: Function

(c,0)
x

Vertical line

23

The Slope-Intercept Form
The Slope-Intercept Form of the
Chapter 1: Function

Equation of a Line

The equation y = mx + b is the equation
of the line whose slope is m and whose
y-intercept is the point (0, b).

24

Example 5
 Find an equation of the line that
passes through the point (5,1) and
Chapter 1: Function

whose slope is equal to1/2.

 Find an equation of the line that
passes through the points (3,-2) and
(1,6).

25

Example 6
Since the beginning of the year, the price
of whole-wheat bread at a local discount
supermarket has been rising at a constant
Chapter 1: Function

rate of 2 cents per month. By November 1,
the price had reached $1.06 per loaf.
Express the price of the bread as a
function of time and determine the price at
the beginning of the year.

26

Example 6 (Cont.)
0 1 2 10
Chapter 1: Function

Jan. 1 Nov. 1

Let x denote the number of months that
have elapsed since January 1 and y
denote the price of a loaf of bread (in
cents).

27

Example 6 (Cont.)
Since y changes at a constant rate with
respect to x, the function relating y to x
must be linear and its graph is a straight
Chapter 1: Function

line. Because the price y increases by 2
each time x increase by 1, the slope of the
line must be 2. Then, we have to write the
equation of the line with slope 2 and
passes through the point . By the formula,
we obtain

28

Example 6 (Cont.)
y y0 m x x0
y 106 2 x 10
Chapter 1: Function

or
y 2 x 86
At the beginning of the year, we
have x 0, then y 86. Hence,
the price of bread at the beginning
of the year was 86 cents per loaf.

29

Example 7
The average scores of incoming students
at an eastern liberal arts college in the SAT
Chapter 1: Function

mathematics examination have been
declining at a constant rate in recent years.
In 1986, the average SAT score was 575,
while in 1991 it was 545.

30

Example 7
- Express the average SAT score as a
function of time.
Chapter 1: Function

- If the trend continues, what will the
average SAT score of incoming students
be in 1996?
- If the trend continues, when will the
average SAT score be 527?

31

Functional Model: Example 1
A manufacturer can produce radios at a cost
of $ 2 apiece. The radios have been selling
for $ 5 apiece, and at this price, consumers
Chapter 1: Function

have been buying 4000 radios a month. The
manufacturer is planning to raise the price
of the radios and estimate that for each $1
increase in the price, 400 fewer radios will
be sold each month. Express the
manufacturer’s monthly profit as a function
of the price at which the radios are sold.

32


Profit (number of radios sold) (Profit per radio)
Chapter 1: Function

o # of radios sold 4000 400(x 85) 400(15 x)
o Profit per radio x 2
Total profit is
P(x) 400(15 x)(x 2)

33

During a drought, residents of Marin Country,
California, were faced with a severe water
shortage. To discourage excessive use of
Chapter 1: Function

water, the country water district initiated
drastic rate increases. The monthly rate for a
family of four was $ 1.22 per 100 cubic feet
of water for the first 1,200 cubic feet, $10 per
100 cubic feet for the next 1200 cubic feet,
and $50 per 100 cubic feet thereafter.
Express the monthly water bill for a family of
four as a function of the amount of water
used. 34

o Let x denote the number of hundred-
cubic-feet units of water used by the
Chapter 1: Function

family during the month C (x) the
corresponding cost in dollars.
o If 0 x 12 the cost is C(x) 1.22 x
o If 12< x 24 the cost is computed by
C(x) 1.22 12 10(x 12) 10x 105.36
o If x >24 the cost is computed by
C(x) 1.22 12 10 12 50(x 24)
C(x) 50x 1,065.36
35

Combining the three equations we
obtain
Chapter 1: Function

1.22 x, if 0 x 12
C x 10 x 105.36 if 12 x 24
50 x 1, 065.36 if x 24

36

Break-Even Analysis
y
Revenue: y R x
Break-even point
Chapter 1: Function

P Profit
Cost: y C x
Loss

0 x

37

Example 8
The Green-Belt Company determines that
the cost of manufacturing men’s belts is $ 2
Chapter 1: Function

each plus $ 300 per day in fixed costs. The
company sells the belts for $ 3 each. What
is the break-even point?

38

Example 9

Suppose that a company has determined
Chapter 1: Function

that the cost of producing x items is
500 140x and that the price it should
charge for one item is p 200 x
a) Find the cost function.
b) Find the revenue function.
c) Find the profit function.
d) Find the break-even point
39

Example 9
a) The cost function is given by
C(x) 500 140x
Chapter 1: Function

b) The revenue function is found by
multiplying the price for one item by the
number of items sold.
R(x) x p(x) x(200 x)
R(x) 200x x2
c) Profit is the difference between
revenue and cost
P(x) R(x) C(x)
40

Example 9
2
P x 200 x x 500 140 x
2
x 60 x 500
Chapter 1: Function

d) To find the break- event, set the revenue
equal to the cost and solve for x
R x C x
2
200 x x 500 140 x
2
x 60 x 500 0
x 10 x 50 0
x 10 or x 50
41

Market Equilibrium
q
Supply: q S p
Chapter 1: Function

Equilibrium

Shortage point
Surplus

Demand: q D p

p p
Equilibrium
price

42

Example 10
Find the equilibrium price and the
corresponding number of units supplied
and demanded if the supply function for a
Chapter 1: Function

certain commodity is S(p) p2 3p 70 and
the demand function is
D(p) = 410 p .

43

Example 10
2
p 3 p 70 140 p
2
p 4 p 480 0
p 20 p 24 0
Chapter 1: Function

p 20 or p 24
Hence we conclude that the equilibrium price is
$20. Since the corresponding supply and
demand are equal, we use the simpler demand
equation to compute this quantity to get
D(20) 410 20 390.
Hence, 390 units are supplied and demanded
when the market is in equilibrium.
44

Thank You
Very Much for
Your Attention!

45

Math for Bus. and Eco. Chapter 1

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Math for Bus. and Eco. Chapter 1