8.1 Ratio and Proportion

8.1 Ratio and Proportion Geometry Mrs. Spitz Spring 2005 Slide #1

Find and simplify the ratio of two numbers. Use proportions to solve real-life problems, such as computing the width of a painting. Chapter 8 Definitions Ch.8 Postulates/ Theorems WS 8.1A Due end of class WS 8.1B Homework Quizzes after 8.3 and 8.5 and 8.7 Objectives/Assignment Slide #2

Computing Ratios • If a and b are two quantities that are measured in the same units, then the ratio of a to be is a/b. The ratio of a to be can also be written as a:b. Because a ratio is a quotient, its denominator cannot be zero. Ratios are usually expressed in simplified form. For instance, the ratio of 6:8 is usually simplified to 3:4. (You divided by 2) Slide #3

Ex. 1: Simplifying Ratios • Simplify the ratios: • 12 cm b. 6 ft c. 9 in. 4 cm 18 ft 18 in. Slide #4

Ex. 1: Simplifying Ratios • Simplify the ratios: • 12 cm b. 6 ft 4 m 18 in Solution: To simplify the ratios with unlike units, convert to like units so that the units divide out. Then simplify the fraction, if possible. Slide #5

Ex. 1: Simplifying Ratios • Simplify the ratios: • 12 cm 4 m 12 cm12 cm123 4 m 4∙100cm 400 100 Slide #6

Ex. 1: Simplifying Ratios • Simplify the ratios: b. 6 ft 18 in 6 ft6∙12 in 72 in. 4 4 18 in 18 in. 18 in. 1 Slide #7

The perimeter of rectangle ABCD is 60 centimeters. The ratio of AB: BC is 3:2. Find the length and the width of the rectangle Ex. 2: Using Ratios Slide #8

SOLUTION: Because the ratio of AB:BC is 3:2, you can represent the length of AB as 3x and the width of BC as 2x. Ex. 2: Using Ratios Slide #9

Statement 2l + 2w = P 2(3x) + 2(2x) = 60 6x + 4x = 60 10x = 60 x = 6 Reason Formula for perimeter of a rectangle Substitute l, w and P Multiply Combine like terms Divide each side by 10 Solution: So, ABCD has a length of 18 centimeters and a width of 12 cm. Slide #10

The measures of the angles in ∆JKL are in the extended ratio 1:2:3. Find the measures of the angles. Begin by sketching a triangle. Then use the extended ratio of 1:2:3 to label the measures of the angles as x°, 2x°, and 3x°. Ex. 3: Using Extended Ratios 2x° 3x° x° Slide #11

Statement x°+ 2x°+ 3x° = 180° 6x = 180 x = 30 Reason Triangle Sum Theorem Combine like terms Divide each side by 6 Solution: So, the angle measures are 30°, 2(30°) = 60°, and 3(30°) = 90°. Slide #12

The ratios of the side lengths of ∆DEF to the corresponding side lengths of ∆ABC are 2:1. Find the unknown lengths. Ex. 4: Using Ratios Slide #13

SOLUTION: DE is twice AB and DE = 8, so AB = ½(8) = 4 Use the Pythagorean Theorem to determine what side BC is. DF is twice AC and AC = 3, so DF = 2(3) = 6 EF is twice BC and BC = 5, so EF = 2(5) or 10 Ex. 4: Using Ratios 4 in a2 + b2 = c2 32 + 42 = c2 9 + 16 = c2 25 = c2 5 = c Slide #14

An equation that equates two ratios is called a proportion. For instance, if the ratio of a/b is equal to the ratio c/d; then the following proportion can be written: Using Proportions Means Extremes  =  The numbers a and d are the extremes of the proportions. The numbers b and c are the means of the proportion. Slide #15

Properties of proportions • CROSS PRODUCT PROPERTY. The product of the extremes equals the product of the means. If  = , then ad = bc Slide #16

Properties of proportions • RECIPROCAL PROPERTY. If two ratios are equal, then their reciprocals are also equal. If  = , then =  b a To solve the proportion, you find the value of the variable. Slide #17

Ex. 5: Solving Proportions 4 5 Write the original proportion. Reciprocal prop. Multiply each side by 4 Simplify. = x 7 4 x 7 4 = 4 5 28 x = 5 Slide #18

Ex. 5: Solving Proportions 3 2 Write the original proportion. Cross Product prop. Distributive Property Subtract 2y from each side. = y + 2 y 3y = 2(y+2) 3y = 2y+4 y 4 = Slide #19

8.1 Ratio and Proportion