Quadratic functions and models

Section 3.1 Quadratic Functions and Models
DEFINITION: A quadratic function is a function f of the form

f (x) = ax2 + bx + c

where a, b, and c are real numbers and a = 0.

Graphing Quadratic Functions Using the Standard Form

EXAMPLE: Let f (x) = 2x2 − 12x + 23.
(a) Express f in standard form.
(b) Sketch the graph of f.

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EXAMPLE: Let f (x) = 2x2 − 12x + 23.
Solution:
(a) We have

f (x) = 2x2 − 12x + 23
= 2(x2 − 6x) + 23
= 2(x2 − 2x · 3) + 23
= 2(x2 − 2x · 3 + 32 − 32 ) + 23
= 2(x2 − 2x · 3 + 32 ) − 2 · 32 + 23
= 2(x − 3)2 + 5

The standard form is f (x) = 2(x − 3)2 + 5.
(b) The standard form tells us that we get the graph of f by taking the parabola y = x2 ,
shifting it to the right 3 units, stretching it by a factor of 2, and moving it upward 5 units. The
vertex of the parabola is at (3, 5), and the parabola opens upward. We sketch the graph in the
Figure below after noting that the y-intercept is f (0) = 23.

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Maximum and Minimum Values of Quadratic Functions

EXAMPLE: Consider the quadratic function f (x) = 5x2 − 30x + 49.
(c) Find the minimum value of f.
Solution:
(a) We have

f (x) = 5x2 − 30x + 49
= 5(x2 − 6x) + 49
= 5(x2 − 2x · 3) + 49
= 5(x2 − 2x · 3 + 32 − 32 ) + 49
= 5(x2 − 2x · 3 + 32 ) − 5 · 32 + 49
= 5(x − 3)2 + 4

(b)

(c)

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(b) The graph is a parabola that has its vertex at (3, 4) and opens upward, as sketched in the
Figure below.

(c) Since the coeﬃcient of x2 is positive, f has a minimum value. The minimum value is
f (3) = 4.


4

Solution:
(a) We have

f (x) = 2x2 − 8x + 13
= 2(x2 − 4x) + 13
= 2(x2 − 2x · 2) + 13
= 2(x2 − 2x · 2 + 22 − 22 ) + 13
= 2(x2 − 2x · 2 + 22 ) − 2 · 22 + 13
= 2(x − 2)2 + 5

(b) The graph is a parabola that has its vertex at (2, 5) and opens upward, as sketched in the
Figure below.

(c) Since the coeﬃcient of x2 is positive, f has a minimum value. The minimum value is
f (2) = 5.

EXAMPLE: Consider the quadratic function f (x) = −x2 + x + 2.
(c) Find the maximum value of f.

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EXAMPLE: Consider the quadratic function f (x) = −x2 + x + 2.
(c) Find the maximum value of f.
Solution:
(a) We have
f (x) = −x2 + x + 2
= −(x2 − x) + 2
( )
1
= − x − 2x ·
2
+2
2
( )
1 1 1
= − x − 2x · + 2 − 2 + 2
2
2 2 2
( )
1 1 1
= − x − 2x · + 2 + 2 + 2
2
2 2 2
( )2
1 9
=− x− +
2 4
( )2
1 9
The standard form is f (x) = − x − + .
2 4
(b) From the standard form we see that the graph is a parabola that opens downward and
( )
has vertex 1 , 9 . As an aid to sketching the graph, we find the intercepts. The y-intercept is
2 4
f (0) = 2. To find the x-intercepts, we set f (x) = 0 and factor the resulting equation.
−x2 + x + 2 = 0
x2 − x − 2 = 0
(x − 2)(x + 1) = 0
Thus, the x-intercepts are x = 2 and x = −1. The graph of f is sketched in the Figure below.

(1) 9
(c) Since the coefficient of x2 is negative, f has a maximum value, which is f 2
= 4.

6

Expressing a quadratic function in standard form helps us sketch its graph as well as ﬁnd its
maximum or minimum value. If we are interested only in ﬁnding the maximum or minimum
value, then a formula is available for doing so. This formula is obtained by completing the
square for the general quadratic function as follows:

f (x) = ax2 + bx + c
( )
2 b
=a x + x +c
a
( )
2 b
= a x + 2x +c
2a
( ( )2 ( )2 )
b b b
= a x2 + 2x + − +c
2a 2a 2a
( ( )2 ) ( )2
b b b
2
= a x + 2x + −a +c
2a 2a 2a
( )2
b b2
=a x+ −a 2 +c
2a 4a
( )2
b b2
=a x+ − +c
2a 4a

EXAMPLE: Find the maximum or minimum value of each quadratic function.
(a) f (x) = x2 + 4x (b) g(x) = −2x2 + 4x − 5

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EXAMPLE: Find the maximum or minimum value of each quadratic function.
(a) f (x) = x2 + 4x (b) g(x) = −2x2 + 4x − 5
Solution:
(a) This is a quadratic function with a = 1 and b = 4. Thus, the maximum or minimum value
occurs at
b 4
x=− =− = −2
2a 2·1
Since a > 0, the function has the minimum value

f (−2) = (−2)2 + 4(−2) = −4

(b) This is a quadratic function with a = −2 and b = 4. Thus, the maximum or minimum value
occurs at
b 4
x=− =− =1
2a 2 · (−2)
Since a < 0, the function has the maximum value

f (1) = −2(1)2 + 4(1) − 5 = −3

Modeling with Quadratic Functions

EXAMPLE: Most cars get their best gas mileage when traveling at a relatively modest speed.
The gas mileage M for a certain new car is modeled by the function
1 2
M (s) = − s + 3s − 31, 15 ≤ s ≤ 70
28
where s is the speed in mi/h and M is measured in mi/gal. What is the car’s best gas mileage,
and at what speed is it attained?

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EXAMPLE: Most cars get their best gas mileage when traveling at a relatively modest speed.
The gas mileage M for a certain new car is modeled by the function
1 2
M (s) = − s + 3s − 31, 15 ≤ s ≤ 70
28
where s is the speed in mi/h and M is measured in mi/gal. What is the car’s best gas mileage,
and at what speed is it attained?
Solution: The function M is a quadratic function with a = − 28 and b = 3. Thus, its maximum
1

value occurs when
b 3
s = − = − ( 1 ) = 42
2a 2 − 28
The maximum is M (42) = − 28 (42)2 + 3(42) − 31 = 32. So the car’s best gas mileage is 32
1

mi/gal, when it is traveling at 42 mi/h.

EXAMPLE: A gardener has 140 feet of fencing to fence in a rectangular vegetable garden. Find
the dimensions of the largest area she can fence.

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EXAMPLE: A gardener has 140 feet of fencing to fence in a rectangular vegetable garden. Find
the dimensions of the largest area she can fence.

Solution: We are asked to find the dimensions of the largest area the gardener can fence. So
we let
x = the width of garden
Then we translate the information in the Figure above into the language of algebra:

The model is the function A that gives the area of the garden for any width x.

area = width × length

A(x) = x(70 − x)
A(x) = 70x − x2

The area she can fence is modeled by the function A(x) = 70x − x2 . We need to find the
maximum value of this function. Since this is a quadratic function with a = −1 and b = 70,
the maximum occurs at
b 70
x=− =− = 35
2a 2(−1)
So the maximum area that she can fence has width 35 ft and length 70 − 35 = 35 ft.

EXAMPLE: If 1800 ft of fencing is available to build five adjacent pens, as shown in the diagram
below, express the total area of the pens as a function of x. What value of x will maximize the
total area?

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EXAMPLE: If 1800 ft of fencing is available to build ﬁve adjacent pens, as shown in the diagram
below, express the total area of the pens as a function of x. What value of x will maximize the
total area?

Solution: The function that models the area will be

A(x) = −3x2 + 900x

where x is the width of the pens. It follows that the area will attain its maximum when
900
x= = 150 ft
2·3
Using this we can ﬁnd that the maximum area will be

A(x) = −3 · 1502 + 900 · 150 = 67, 500 ft

EXAMPLE: A hockey team plays in an arena that has a seating capacity of 15,000 spectators.
With the ticket price set at $14, average attendance at recent games has been 9500. A market
survey indicates that for each dollar the ticket price is lowered, the average attendance increases
by 1000.
(a) Find a function that models the revenue in terms of ticket price.
(b) What ticket price is so high that no one attends, and hence no revenue is generated?
(c) Find the price that maximizes revenue from ticket sales.

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EXAMPLE: A hockey team plays in an arena that has a seating capacity of 15,000 spectators.
With the ticket price set at $14, average attendance at recent games has been 9500. A market
survey indicates that for each dollar the ticket price is lowered, the average attendance increases
by 1000.
(a) Find a function that models the revenue in terms of ticket price.
(b) What ticket price is so high that no one attends, and hence no revenue is generated?
(c) Find the price that maximizes revenue from ticket sales.
Solution:
(a) The model we want is a function that gives the revenue for any ticket price. We know that
revenue = ticket price × attendance
There are two varying quantities: ticket price and attendance. Since the function we want
depends on price, we let
x = ticket price
Next, we must express the attendance in terms of x.

The model is the function R that gives the revenue for a given ticket price x.
revenue = ticket price × attendance

R(x) = x(23, 500 − 1000x)
R(x) = 23, 500x − 1000x2
We use the model to answer the questions in parts (b) and (c).
(b) We want to ﬁnd the ticket price x for which

R(x) = 23, 500x − 1000x2 = 0

We can solve this quadratic equation algebraically or graphically.
From the graph in the Figure we see that R(x) = 0 when x = 0 or
x = 23.5. So, according to our model, the revenue would drop to
zero if the ticket price is $23.50 or higher. (Of course, revenue is
also zero if the ticket price is zero!)

(c) Since R(x) = 23, 500x − 1000x2 is a quadratic function with a = −1000 and b = 23, 500,
the maximum occurs at
b 23, 500
x=− =− = 11.75
2a 2(−1000)
So a ticket price of $11.75 yields the maximum revenue. At this price the revenue is
R(11.75) = 23, 500(11.75) − 1000(11.75)2 = $138, 062.50

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Quadratic functions and models

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