BASIC GEOMETRY
7.1: RATIO & PROPORTION
Ratio
Compares two numbers by division.
Writing ratios
a
to b
a:b
a
b
Slope = ratio of rise/run
Write a ratio expressing the slope of l.
Extended Ratios
Compares more than 2 numbers.
Example: For the rectangle, the ratio of the side
lengths may be written as 3:7:3:7
Extended Ratio Example
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?
Proportion
Equation stating that two ratios are equal.
a c
or a:b = c:d
Solving Proportions
b
d
a&d are the extremes
b&c are the means
Use
the cross products property (means = extremes)
a c
→ ad = bc
b d
Hints: When multiplying sums or differences, use
parenthesis.
Solve each proportion.
A) 7 56
x 72
20
B) x 4
5
x4
Equivalent Forms of Cross Products
Using Properties of Proportions
Given that 18c = 24d, find the ratio of d to c in
simplest form.
Example
Marta is making a scale drawing of her bedroom.
Her rectangular room is 12½ feet wide and 15
feet long. On the scale drawing, the width of her
room is 5 inches. What is the length?
Assignment 1
Page 457 #’s 3-10,14-16,(56)
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