7-1 Ratios and ProportionsI CAN•Write a ratio•Write a ratio expressing the slope of a line.•Solve a linear proportion•Solve a quadratic proportion•Use a proportion to determine if a figure has been dilated.

A ratio compares two numbers by division. The ratio

of two numbers a and b can be written as a to b, a:b,

or , where b ≠ 0. For example, the ratios 1 to 2,

1:2, and all represent the same comparison.

Example: There are 11 boys and 15 girls in class. Write the ratio of girls to boys.

15 to 11 15:11 1511

The order of the numbers matters!

Writing Ratios to Express Slope of a Line

In Algebra I, you learned that the slope of a line (m) is an example of a ratio. Slope is a rate of change and can be expressed in the following ways:

rise run

yx

y2 – y1

x2 – x1

Writing Ratios to Express Slope of a Line

Write a ratio expressing the slope of the give line.

Substitute the given values.

Simplify.

Ratios in Similar PolygonsA ratio can involve more than two numbers. For the rectangle, the ratio of the side lengths may be written as 3:7:3:7.

Example : Using Ratios

The ratio of the side lengths of a triangle is 4:7:5, and its perimeter is 96 cm. What is the length of the shortest side?

Let the side lengths be 4x, 7x, and 5x.

4x + 7x + 5x = 96 16x = 96 x = 6

The length of the shortest side is 4x = 4(6) = 24 cm.

A proportion is an equation stating that two ratios are equal to each other.

In a proportion, the cross products ad and bc are equal.

Solving Linear Proportions

To solve a proportion, “CROSS MULTIPLY AND SIMPLIFY.”

Example

4 = k10 65

10k = 260 Cross multiply

10k = 260 10 10

Simplify by dividing both sides of equation by 10

k = 26

Example

3 = 4(x + 3) (x + 8)

3(x + 8) = 4(x + 3) Cross multiply

3x + 24 = 4x + 12 Simplify by distributing-3x -3x Get variable on same side of equation

24 = x + 12-12 – 12 12 = x

Solving Linear Proportions

Your Turn

7 = 2 3x (x + 4)

x = -28

Solving Linear Proportions

Solving Quadratic Proportions

Example

2y = 8 9 4y

8y2 = 72 Cross multiply

8 8

y2 = 9

392

yy

Simplify

Take the positive and negative square root of both sides

Solving Quadratic ProportionsYour Turn

14 = 2x x 7

7x

Solving Quadratic ProportionsExample

(x+3) = 9 4 (x+3)

(x+3)(x+3) = 36 Cross multiply

x2 + 6x + 9 = 36 FOIL -36 -36

x2 + 6x – 27 = 0 Solve quadratic equations by setting equation = 0

( x – 3 )( x + 9 ) = 0 Factorx -3 = 0 x + 9 =0

x = 3 x = -9

Use Zero Product Property to find solutions

Solving Quadratic ProportionsYour Turn

(x – 4) = 20 5 (x – 4)

x = 14 x = -6

Solving Quadratic ProportionsExample

3 = (x – 8)(x + 9) (3x – 8)

3(3x – 8) = (x – 8)(x + 9)

9x – 24 = x2 + 9x – 8x – 72

9x – 24 = x2 + x – 72

– 9x + 24 – 9x + 24

0 = x2 – 8x – 48 0 = (x – 12)(x + 4)

x – 12 = 0 x + 4 = 0x = 12 or x = – 4

Dilations and Proportions

When a figure is dilated, the pre-image and image are proportional.

You can use proportions to find missing measures and to check dilations!

Refer to the “Dilations as Proportions” Worksheet in your Unit plan.We will now work examples 1 and 2.

Dilations as Proportions

C U

TE

8 cm3 cm

U G

LY

7.5 cm

Ex) Rectangle CUTE was dilated to create rectangle UGLY. Find the length of LY.

3 = 87.5 UG

Pre-image and image of dilated figures are proportional

3 = 87.5 LY

Opposite sides of a rectangle are congruent.

3LY = 8(7.5)

3LY = 60

LY = 20 cm

Cross multiply

Simplify

Dilations as Proportions

6 in.

2.25 in.

20 in.

10 in.

8 in.

3 in.

30 in.

5 in.

A B C D

Ex) Determine which of the following figures could be a dilation of the triangle onthe right (There could be more than one answer!)

16 in.

6 in.

Triangle A

6 = 2.2516 6

36 = 2.25(16)?36 = 36? YES

Triangle B

20 = 10 16 6

20(6) = 10(16)?120 = 160? NO

Triangle C

8 = 3 16 6

8(6)=16(3)?48 = 48?YES

Triangle D

30 = 5 16 6

30(6) =16(5)?180 = 80? NO

Now complete #1 & 2 on Dilations as Proportions Worksheet