What is a scale factor and how do you compute one?

Scale in miles:1 inch = Approx. 15 miles

Funville

Gladberg

Happy Town Smile

City

In this lesson you will learn how to determine the scale factor between two similar figures by applying ratios.

Let’s ReviewLet’s ReviewA ratio is a comparison between

two numbers.

For example:# Apples : # Oranges = 4 : 2 or 2 : 1

Let’s ReviewA Common MistakeReversing the order of the ratio

For example:# Apples : # Oranges = 2 : 4 or 1 : 2 -

Let’s ReviewCore LessonTwo figures are similar if the lengths of every pair of corresponding sides are in

the same ratio.

A

B

C

L

M

N

16 22

24

8 11

12

=

=

=

=

=

=

Let’s ReviewCore LessonThe scale factor is the ratio of the lengths of

corresponding sides in similar figures.Q

T S

R4

4

6

YZ

XW

6

Scale factor of small to large = = Scale factor of large to small = =

Let’s ReviewCore LessonScale factor on a map:

Scale in miles:1 inch = Approx.

15 miles

Funville

Gladberg

Happy Town

Smile City

The number of inches on the

map that corresponds to

the actual number of miles

45 miles

3 inches

In this lesson you have learned how to determine the scale factor between two similar figures by applying ratios.

Let’s ReviewGuided Practice

Find the scale factor of the small triangle to the large triangle.

1010

16

25 25

40

Let’s ReviewExtension Activities

Describe in words what it means when a diagram is drawn in a 1:10 scale.

Let’s ReviewExtension ActivitiesFind the scale factor of triangle DEF to triangle PQR.

D

E

F

P

Q

R42

9 15

Perimeter of PQR = 42

Let’s ReviewQuick Quiz

1. Find the scale factor of the small rectangle to the large rectangle.

80

30

12

32

Let’s ReviewQuick Quiz

2. Find the scale factor of the large triangle to the small triangle.

AB

C

L

M

N

26

45

14

20