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Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Solve quadratic equations by graphing or factoring.Determine a quadratic function from its roots.

Objectives

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

A zero of a function is a value of the input x that makes the output f(x) equal zero. The zeros of a function are the x-intercepts.

Unlike linear functions, which have no more than one zero, quadratic functions can have two zeros, as shown at right. These zeros are always symmetric about the axis of symmetry.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the zeros of f(x) = x2 – 6x + 8 by using a graph and table.

Example 1: Finding Zeros by Using a Graph or Table

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the zeros of f(x) = x2 – 6x + 8 by using a graph and table.

Example 1 Continued

Method 2 Use a calculator.

Enter y = x2 – 6x + 8 into a graphing calculator.

Both the table and the graph show that y = 0 at x = 2 and x = 4. These are the zeros of the function.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Check It Out! Example 1

Find the zeros of g(x) = –x2 – 2x + 3 by using a graph and a table.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the zeros of f(x) = –x2 – 2x + 3 by using a graph and table.

Method 2 Use a calculator.

Enter y = –x2 – 2x + 3 into a graphing calculator.

Both the table and the graph show that y = 0 at x = –3 and x = 1. These are the zeros of the function.

Check It Out! Example 1 Continued

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

You can also find zeros by using algebra. For example, to find the zeros of f(x)= x2 + 2x – 3, you can set the function equal to zero. The solutions to the related equation x2 + 2x – 3 = 0 represent the zeros of the function.

The solution to a quadratic equation of the form ax2 + bx + c = 0 are roots. The roots of an equation are the values of the variable that make the equation true.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

You can find the roots of some quadratic equations by factoring and applying the Zero Product Property.

• Functions have zeros or x-intercepts.

• Equations have solutions or roots.

Reading Math

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the zeros of the function by factoring.

Example 2A: Finding Zeros by Factoring

f(x) = x2 – 4x – 12

x2 – 4x – 12 = 0

(x + 2)(x – 6) = 0

x + 2 = 0 or x – 6 = 0

x= –2 or x = 6

Set the function equal to 0.

Factor: Find factors of –12 that add to –4.

Apply the Zero Product Property.

Solve each equation.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the zeros of the function by factoring.

Example 2B: Finding Zeros by Factoring

g(x) = 3x2 + 18x

3x2 + 18x = 0

3x(x+6) = 0

3x = 0 or x + 6 = 0

x = 0 or x = –6

Set the function to equal to 0.

Factor: The GCF is 3x.

Apply the Zero Product Property.

Solve each equation.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

A golf ball is hit from ground level with an initial vertical velocity of 80 ft/s. After how many seconds will the ball hit the ground?

Example 3: Sports Application

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Example 3 Continued

The ball will hit the ground when its height is zero.

–16t2 + 80t = 0

–16t(t – 5) = 0

–16t = 0 or (t – 5) = 0

t = 0 or t = 5

Set h(t) equal to 0.

Factor: The GCF is –16t.

Apply the Zero Product Property.

Solve each equation.

The golf ball will hit the ground after 5 seconds. Notice that the height is also zero when t = 0, the instant that the golf ball is hit.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Check The graph of the function h(t) = –16t2 + 80t shows its zeros at 0 and 5.

Example 3 Continued

–15

105

7–3

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the roots of the equation by factoring.

Example 4A: Find Roots by Using Special Factors

4x2 = 25

Rewrite in standard form.

Factor the difference of squares.

Write the left side as a2 – b2.

Apply the Zero Product Property.

Solve each equation.

4x2 – 25 = 0

(2x)2 – (5)2 = 0

(2x + 5)(2x – 5) = 0

2x + 5 = 0 or 2x – 5 = 0

x = – or x =

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Find the roots of the equation by factoring.

Example 4B: Find Roots by Using Special Factors

18x2 = 48x – 32

Rewrite in standard form.

Factor. The GCF is 2.

Divide both sides by 2.

Write the left side as a2 – 2ab +b2.

Apply the Zero Product Property.

Solve each equation.

18x2 – 48x + 32 = 0

2(9x2 – 24x + 16) = 0

(3x – 4)2 = 0

3x – 4 = 0 or 3x – 4 = 0

x = or x =

9x2 – 24x + 16 = 0

(3x)2 – 2(3x)(4) + (4)2 = 0

Factor the perfect-square trinomial.

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Write a quadratic function in standard form with zeros 4 and –7.

Example 5: Using Zeros to Write Function Rules

Write the zeros as solutions for two equations.

Rewrite each equation so that it equals 0.

Apply the converse of the Zero Product Property to write a product that equals 0.

Multiply the binomials.

x = 4 or x = –7

x – 4 = 0 or x + 7 = 0

(x – 4)(x + 7) = 0

x2 + 3x – 28 = 0

f(x) = x2 + 3x – 28 Replace 0 with f(x).

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Check It Out! Example 5

Write a quadratic function in standard form with zeros 5 and –5.

x = 5 or x = –5

x + 5 = 0 or x – 5 = 0

(x + 5)(x – 5) = 0

x2 – 25 = 0

f(x) = x2 – 25

Write the zeros as solutions for two equations.

Rewrite each equation so that it equals 0.

Apply the converse of the Zero Product Property to write a product that equals 0.

Multiply the binomials.

Replace 0 with f(x).

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Note that there are many quadratic functions with the same zeros. For example, the functions f(x) = x2 – x – 2, g(x) = –x2 + x + 2, and h(x) = 2x2 – 2x – 4 all have zeros at 2 and –1.

–5

–7.6

5

7.6

Holt McDougal Algebra 2

5-3 Solving Quadratic Equations by Graphing and Factoring

Lesson Quiz: Part I

Find the zeros of each function.

2. f(x) = x2 – 9x + 20

1. f(x)= x2 – 7x 0, 7

3. x2 – 10x + 25 = 0

4, 5

Find the roots of each equation using factoring.

4. 7x = 15 – 2x2

5

–5,