grade 8 algebra1 solving quadratic equations by graphing

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CONFIDENTIAL 1 Grade 8 Algebra1 Grade 8 Algebra1 Solving Solving Quadratic Quadratic Equations by Equations by Graphing Graphing

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Grade 8 Algebra1 Solving Quadratic Equations by Graphing. Warm Up. Write an equation in point-slope form for the line with the given slope that contains the given point. 1) slope = -3; (-2, 4). 2) slope = 0; (2, 1). 3) slope = 1 ; (2, 3) 2. 2) y = 1. 1) y - 4= -3(x + 2). - PowerPoint PPT Presentation

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Page 1: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 1

Grade 8 Algebra1Grade 8 Algebra1

Solving Quadratic Solving Quadratic Equations by Equations by

GraphingGraphing

Page 2: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 2

Warm UpWarm Up

Write an equation in point-slope form for the line with the given slope that contains the given point.

1) slope = -3; (-2, 4)

2) slope = 0; (2, 1)

1) y - 4= -3(x + 2)

2) y = 1

3) slope = 1; (2, 3) 2

3) y - 3= 1(x -2) 2

Page 3: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 3

Every quadratic function has a related quadraticequation. A quadratic equation is an equation

that can be written in the standard form ax2 + bx + c = 0,

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

Solving Quadratic Equations by GraphingSolving Quadratic Equations by Graphing

Notice that when writing a quadratic function as its related quadratic equation,

you replace y with 0. So y = 0.

y = ax2 + bx + c

0 = ax2 + bx + c

ax2 + bx + c = 0

Page 4: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 4

One way to solve a quadratic equation in standard form is to graph the related function and find the x-values where y = 0. In other words, find the zeros of the related function. Recall that a quadratic function

may have two, one, or no zeros.

Solving Quadratic Equations by Graphing

Step 1 Write the related function.

Step 2 Graph the related function.

Step 3 Find the zeros of the related function.

Page 5: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 5

Solving Quadratic Equations by GraphingSolving Quadratic Equations by Graphing

Solve each equation by graphing the related function.

A) 2x2 - 2 = 0

Step 1 Write the related function.

2x2 - 2 = y, or y = 2x2 + 0x - 2

Step 2 Graph the related function.

• The axis of symmetry is x = 0.

• The vertex is (0, -2) .

• Two other points are (1, 0) and (2, 6).

• Graph the points and reflect them across the axis of symmetry.

Page 6: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 6

Step 3 Find the zeros of the related function.

The zeros appear to be -1 and 1.

Check

Substitute -1 and 1 for x in the quadratic equation.

2x2 - 2 = 0

2-(1)2 - 2 02(1) – 2 0

2(1) – 2 00 0

2x2 - 2 = 0

2(1)2 - 2 02(1) – 2 0

2(1) – 2 00 0

Page 7: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 7

Solve each equation by graphing the related function.

B) -x2 - 4x - 4 = 0

Step 1 Write the related function.

y = -x2 - 4x - 4 = 0

Step 2 Graph the related function.

• The axis of symmetry is x = -2.

• The vertex is (-2, 0).

• The y-intercept is -4.

• Another point is (-1, -1).

• Graph the points and reflect them across the axis of symmetry.

Page 8: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 8

Step 3 Find the zeros of the related function.

The only zero appears to be -2.

Check

y = - x2 - 4x - 4

0- -(2)2- 4-(2 )-40 - (4) + 8 -40 -4 + 40 0

Page 9: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 9

C) x2 + 5 = 4x

Step 1 Write the related function.

Step 2 Graph the related function Use a graphing calculator.

x2 - 4x + 5 = 0

y = x2 - 4x + 5

Step 3 Find the zeros of the related function.

The function appears to have no zeros.The equation has no real-number solutions.

Page 10: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 10

Now you try!

Solve each equation by graphing the related function.

1a. x2 - 8x - 16 = 2x2

1b. 6x + 10 = - x2

1c. -x2 + 4 = 0

1a. x = -41b. No zeros1c. x = -2 or x = 2

Page 11: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 11

Aquatics ApplicationAquatics Application

A dolphin jumps out of the water. The quadratic function y = -16x2 + 20x models the dolphin’s height above the water after x seconds. About how long is

the dolphin out of the water?

When the dolphin leaves the water, its height is 0, and when the dolphin reenters the water, its height is

0. So solve 0 = -16x2 + 20x to find the times when the dolphin leaves and reenters the water.

Step 1 Write the related function.

o = -16x2 + 20x y = -16x2 + 20x

Page 12: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 12

Step 3 Find the zeros of the related function.

x = 1.25 y = 0

y = -16x2 + 20x

Step 2 Graph the related function. Use a graphing calculator.

The zeros appear to be 0 and 1.25. The dolphin leaves the water at 0 seconds and reenters the water at 1.25 seconds.

Check

0 - =16x2 + 20x

0- 16(1.25)2- 20(1.25)0 -16(1.5625) + 250 -25 + 250 0

Substitute 1.25 for x in the quadratic equation.

Page 13: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 13

Now you try!

2) Another dolphin jumps out of the water. Thequadratic function y = -16x2 + 32x models the

dolphin’s height above the water after x seconds. About how long is the dolphin out of the water?

2) The dolphin leaves the water at 0 seconds and reenters the water at 2 seconds.

Page 14: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 14

BREAK

Page 15: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 15

Assessment

1) x2 - 4 = 0

Solve each equation by graphing the related function.

2) x2 = 16

3) -2x2 - 6 = 0

1) x = -2, 22) x = -4, 43) x = No real solutions

Page 16: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 16

4) - x2 + 12x - 36 = 0

Solve each equation by graphing the related function.

6) 2x2 = 3x2 - 2x - 8

5) - x2 = -9

4)x = 65) x = -3, 36) x = -2, 4

Page 17: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 17

Solve each equation by graphing the related function.

7) x2 - 6x + 9 = 0

8) 8x = -4x2 - 4

9) x2 + 5x + 4 = 0

7)x = 3, -18) x = -1, 49) x = -4, -1

Page 18: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 18

10) A baseball coach uses a pitching machine to simulate pop flies during practice. The baseball is shot out of the pitching machine with a velocity of 80 feet per second. The quadratic function y = -16x2 + 80x

models the height of the baseball after x seconds. How long is the baseball in the air?

10) The baseball is in the air for 5 seconds.

Page 19: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 19

Let’s review

Every quadratic function has a related quadraticequation. A quadratic equation is an equation

that can be written in the standard form ax2 + bx + c = 0,

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

Solving Quadratic Equations by GraphingSolving Quadratic Equations by Graphing

Notice that when writing a quadratic function as its related quadratic equation,

you replace y with 0. So y = 0.

y = ax2 + bx + c

0 = ax2 + bx + c

ax2 + bx + c = 0

Page 20: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 20

One way to solve a quadratic equation in standard form is to graph the related function and find the x-values where y = 0. In other words, find the zeros of the related function. Recall that a quadratic function

may have two, one, or no zeros.

Solving Quadratic Equations by Graphing

Step 1 Write the related function.

Step 2 Graph the related function.

Step 3 Find the zeros of the related function.

Page 21: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 21

Solving Quadratic Equations by GraphingSolving Quadratic Equations by Graphing

Solve each equation by graphing the related function.

A) 2x2 - 2 = 0

Step 1 Write the related function.

2x2 - 2 = y, or y = 2x2 + 0x - 2

Step 2 Graph the related function.

• The axis of symmetry is x = 0.

• The vertex is (0, -2) .

• Two other points are (1, 0) and (2, 6).

• Graph the points and reflect them across the axis of symmetry.

Page 22: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 22

Step 3 Find the zeros of the related function.

The zeros appear to be -1 and 1.

Check

Substitute -1 and 1 for x in the quadratic equation.

2x2 - 2 = 0

2-(1)2 - 2 02(1) – 2 0

2(1) – 2 00 0

2x2 - 2 = 0

2(1)2 - 2 02(1) – 2 0

2(1) – 2 00 0

Page 23: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 23

Aquatics ApplicationAquatics Application

A dolphin jumps out of the water. The quadratic function y = -16x2 + 20x models the dolphin’s height above the water after x seconds. About how long is

the dolphin out of the water?

When the dolphin leaves the water, its height is 0, and when the dolphin reenters the water, its height is

0. So solve 0 = -16x2 + 20x to find the times when the dolphin leaves and reenters the water.

Step 1 Write the related function.

o = -16x2 + 20x y = -16x2 + 20x

Page 24: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 24

Step 3 Find the zeros of the related function.

x = 1.25 y = 0

y = -16x2 + 20x

Step 2 Graph the related function. Use a graphing calculator.

The zeros appear to be 0 and 1.25. The dolphin leaves the water at 0 seconds and reenters the water at 1.25 seconds.

Check

o = -16x2 + 20x

0- 16(1.25)2- 20(1.25)0 -16(1.5625) + 250 -25 + 250 0

Substitute 1.25 for x in the quadratic equation.

Page 25: Grade 8 Algebra1 Solving Quadratic Equations by Graphing

CONFIDENTIAL 25

You did a great job You did a great job today!today!