10.4 Graphing Quadratic Equations - McGraw Hill Education

© 2001 McGraw-Hill Companies 10.4 Graphing Quadratic Equations

10.4 OBJECTIVE
1. Graph a quadratic equation by plotting points

In Section 6.3 you learned to graph first-degree equations. Similar methods will allow you
to graph quadratic equations of the form
y ϭ ax2 ϩ bx ϩ c a 0
The first thing you will notice is that the graph of an equation in this form is not a straight
line. The graph is always the curve called a parabola.

Here are some examples:

y

x

y

x

To graph quadratic equations, start by finding solutions for the equation. We begin
by completing a table of values. This is done by choosing any convenient values for x.
Then use the given equation to compute the corresponding values for y, as Example 1
illustrates.

Example 1
Completing a Table of Values
If y ϭ x 2, complete the ordered pairs to form solutions. Then show these results in a table
of values.
(Ϫ2, ), (Ϫ1, ), (0, ), (1, ), (2, )

777

778 CHAPTER 10 QUADRATIC EQUATIONS

NOTE Remember that a For example, to complete the pair (Ϫ2, ), substitute Ϫ2 for x in the given equation.
solution is a pair of values that
makes the equation a true y ϭ (Ϫ2)2 ϭ 4
statement.
So (Ϫ2, 4) is a solution.
Substituting the other values for x in the same manner, we have the following table of

values for y ϭ x2:

xy

Ϫ2 4
Ϫ1 1

00
11
24

CHECK YOURSELF 1
If y ‫ ؍‬x2 ؉ 2, complete the ordered pairs to form solutions and form a table of values.

(Ϫ2, ), (Ϫ1, ), (0, ), (1, ), (2, )

We can now plot points in the cartesian coordinate system that correspond to solutions
to the equation.

Example 2
Plotting Some Solution Points
Plot the points from the table of values corresponding to y ϭ x2 from Example 1.

y

xy (Ϫ2, 4) (2, 4)

Ϫ2 4 (Ϫ1, 1) (1, 1)
Ϫ1 1
(0, 0) x
00
11 © 2001 McGraw-Hill Companies
24

Notice that the y axis acts as a mirror. Do you see that any point graphed in quadrant I will
be “reflected” in quadrant II?

GRAPHING QUADRATIC EQUATIONS SECTION 10.4 779

CHECK YOURSELF 2
Plot the points from the table of values formed in Check Yourself 1.

y

x

The graph of the equation can be drawn by joining the points with a smooth curve.

Example 3

Completing the Graph of the Solution Set

Draw the graph of y ϭ x2.

We can now draw a smooth curve between the points found in Example 2 to form the
graph of y ϭ x2.

y

NOTE As we mentioned earlier,
the graph must be the curve
called a parabola.

NOTE Notice that a parabola x
does not come to a point.

© 2001 McGraw-Hill Companies CHECK YOURSELF 3
Draw a smooth curve between the points plotted in the Check Yourself 2 exercise.

y

x

780 CHAPTER 10 QUADRATIC EQUATIONS

You can use any convenient values for x in forming your table of values. You should use
as many pairs as are necessary to get the correct shape of the graph (a parabola).

Example 4
Graphing the Solution Set
Graph y ϭ x2 Ϫ 2x. Use values of x between Ϫ1 and 3.

First, determine solutions for the equation. For instance, if x ϭ Ϫ1,

y ϭ (Ϫ1)2 Ϫ 2(Ϫ1)
ϭ1ϩ2
ϭ3

then (Ϫ1, 3) is a solution for the given equation.
Substituting the other values for x, we can form the table of values shown below. We

then plot the corresponding points and draw a smooth curve to form our graph.

The graph of y ϭ x2 Ϫ 2x.

NOTE Any values can be xy y
substituted for x in the original x
equation. Ϫ1 3
00
1 Ϫ1
20
33

CHECK YOURSELF 4 © 2001 McGraw-Hill Companies
Graph y ‫ ؍‬x 2 ؉ 4x. Use values of x between ؊4 and 0.

y

xy

x

GRAPHING QUADRATIC EQUATIONS SECTION 10.4 781

Choosing values for x is also a valid method of graphing a quadratic equation that
contains a constant term.

Example 5
Graphing the Solution Set

Graph y ϭ x2 Ϫ x Ϫ 2. Use values of x between Ϫ2 and 3. We’ll show the computation for
two of the solutions.

If x ϭ Ϫ2: If x ϭ 3:

y ϭ (Ϫ2)2 Ϫ (Ϫ2) Ϫ 2 y ϭ 32 Ϫ 3 Ϫ 2
ϭ4ϩ2Ϫ2 ϭ9Ϫ3Ϫ2
ϭ4 ϭ4

You should substitute the remaining values for x into the given equation to verify the other
solutions shown in the table of values below.

The graph of y ϭ x2 Ϫ x Ϫ 2.

y

xy x

Ϫ2 4
Ϫ1 0

0 Ϫ2
1 Ϫ2
20
34

© 2001 McGraw-Hill Companies CHECK YOURSELF 5
Graph y ‫ ؍‬x 2 ؊ 4x ؉ 3. Use values of x between ؊1 and 4.

y

xy

x

In Example 6, the graph looks significantly different from previous graphs.

782 CHAPTER 10 QUADRATIC EQUATIONS

Example 6
Graphing the Solution Set

Graph y ϭ Ϫx2 ϩ 3. Use x values between Ϫ2 and 2.
Again we’ll show two computations.

NOTE Ϫ(Ϫ2)2 ϭ Ϫ4 If x ϭ Ϫ2: If x ϭ 1:
y ϭ Ϫ(Ϫ2)2 ϩ 3 y ϭ Ϫ(1)2 ϩ 3

ϭ Ϫ4 ϩ 3 ϭ Ϫ1 ϩ 3
ϭ Ϫ1 ϭ2

Verify the remainder of the solutions shown in the table of values below for yourself.

The graph of y ϭ Ϫx2 ϩ 3.

xy y
x
Ϫ2 Ϫ1
Ϫ1 2

03
12
2 Ϫ1

There is an important difference between this graph and the others we have seen. This © 2001 McGraw-Hill Companies
time the parabola opens downward! Can you guess why? The answer is in the coefficient of
the x2 term.

If the coefficient of x2 is positive, the parabola opens upward.

y

The coefficient of x2 is positive.
x

GRAPHING QUADRATIC EQUATIONS SECTION 10.4 783

If the coefficient of x2 is negative, the parabola opens downward.

y

The coefficient of x2 is negative.
x

© 2001 McGraw-Hill Companies CHECK YOURSELF 6
Graph y ‫ ؍‬؊x 2 ؊ 2x. Use x values between ؊3 and 1.

y

xy

x

There are two other terms we would like to introduce before closing this section on
graphing quadratic equations. As you may have noticed, all the parabolas that we graphed
are symmetric about a vertical line.This is called the axis of symmetry for the parabola.

The point at which the parabola intersects that vertical line (this will be the lowest—or
the highest—point on the parabola) is called the vertex. You’ll learn more about finding the
axis of symmetry and the vertex of a parabola in your next course in algebra.

y The axis of symmetry

x

The vertex

784 CHAPTER 10 QUADRATIC EQUATIONS

CHECK YOURSELF ANSWERS

1. x y 2. y
x
Ϫ2 6
Ϫ1 3

02
13
26

3. y ϭ x2 ϩ 2 4. y ϭ x2 ϩ 4x y
x
y xy
Ϫ4 0
x Ϫ3 Ϫ3
Ϫ2 Ϫ4
Ϫ1 Ϫ3

00

5. y ϭ x2 Ϫ 4x ϩ 3 y © 2001 McGraw-Hill Companies
x
xy
Ϫ1 8 y
x
03
10
2 Ϫ1
30
43

6. y ϭ Ϫx2 Ϫ 2x

xy
Ϫ3 Ϫ3
Ϫ2 0
Ϫ1 1

00
1 Ϫ3

10.4 Exercises Name Date
Section

Graph each of the following quadratic equations after completing the given table of values.

1. y ϭ x2 ϩ 1 ANSWERS
1.
xy y 2.
x 3.
Ϫ2 4.
Ϫ1

0
1
2

2. y ϭ x2 Ϫ 2 y
x
xy
Ϫ2
Ϫ1

0
1
2

3. y ϭ x2 Ϫ 4 y
x
xy
Ϫ2
Ϫ1

0
1
2

© 2001 McGraw-Hill Companies 4. y ϭ x2 ϩ 3 y
x
xy
Ϫ2
Ϫ1

0
1
2

785

ANSWERS 5. y ϭ x2 Ϫ 4x y © 2001 McGraw-Hill Companies
5. x
6. xy
7. 0 y
8. 1 x
2
786 3 y
4 x

6. y ϭ x2 ϩ 2x y
x
xy
Ϫ3
Ϫ2
Ϫ1

0
1

7. y ϭ x 2 ϩ x

xy
Ϫ2
Ϫ1

0
1
2

8. y ϭ x2 Ϫ 3x

xy
Ϫ1

0
1
2
3

© 2001 McGraw-Hill Companies 9. y ϭ x2 Ϫ 2x Ϫ 3 y ANSWERS
x 9.
xy 10.
Ϫ1 y 11.
x 12.
0
1 y 787
2 x
3
y
10. y ϭ x 2 Ϫ 5x ϩ 6 x

xy
0
1
2
3
4

11. y ϭ x 2 Ϫ x Ϫ 6

xy
Ϫ1

0
1
2
3

12. y ϭ x 2 ϩ 3x Ϫ 4

xy
Ϫ4
Ϫ3
Ϫ2
Ϫ1

0

ANSWERS 13. y ϭ Ϫx2 ϩ 2 y © 2001 McGraw-Hill Companies
13. x
14. xy
15. Ϫ2 y
16. Ϫ1 x

788 0 y
1 x
2
y
14. y ϭ Ϫx 2 Ϫ 2 x

xy
Ϫ2
Ϫ1

0
1
2

15. y ϭ Ϫx 2 Ϫ 4x

xy
Ϫ4
Ϫ3
Ϫ2
Ϫ1

0

16. y ϭ Ϫx 2 ϩ 2x

xy
Ϫ1

0
1
2
3

Match each graph with the correct equation on the right. ANSWERS
17.
17. y 18. y 18.
19.
x 20.
21.
19. y 20. y x 22.
23.
x 24.

21. y 22. y (a) y ϭ Ϫx2 ϩ 1
x (b) y ϭ 2x
x
(c) y ϭ x2 Ϫ 4x
23. y 24. y (d) y ϭ Ϫx ϩ 1
(e) y ϭ Ϫx2 ϩ 3x
© 2001 McGraw-Hill Companies x (f ) y ϭ x2 ϩ 1

x

(g) y ϭ x ϩ 1
(h) y ϭ 2x 2

x

789

Answers 3. y ϭ x 2 Ϫ 4

1. y ϭ x 2 ϩ 1

y y

xx

5. y ϭ x 2 Ϫ 4x 7. y ϭ x 2 ϩ x

y y

xx

9. y ϭ x 2 Ϫ 2x Ϫ 3 11. y ϭ x 2 Ϫ x Ϫ 6

y y

xx

13. y ϭ Ϫx 2 ϩ 2 15. y ϭ Ϫx 2 Ϫ 4x

y y

xx © 2001 McGraw-Hill Companies

17. f 19. a 21. b 23. e
790