How could we find the amount of wood needed to frame a poster

made from a photo?

In this lesson you will learn how to compute an actual

perimeter from a scale drawing by using ratios and

proportions.

Let’s ReviewLet’s Review

A proportion is an equation that relates two equal ratios.

For example:

=

Let’s ReviewLet’s Review

Find the length of TQ by cross multiplying

Q

T S

R6

?

10

YZ

XW

15

(6)(15)

  

= (10)(TQ) 90 = (10)(TQ) TQ = 9

Let’s ReviewA Common Mistake

=

Mixing up the order of one ratio in a proportion

A

B

C

L

M

N

16 22

24

811

12

 

-

Let’s ReviewCore Lesson

Ratio of perimeters

A B

C

3

4

D

Q R

S

6

8

T

Perimeter = 14

P = 4 + 3 + 4 + 3 Perimeter = 28P = (2 x 4) + (2 x 3)

P = 8 + 6 + 8 + 6

P = (2 x 8) + (2 x 6)

Let’s ReviewCore Lesson

PERIMETER

Perimeter of LMN = ?

  =

perimeter = 52.5A

B

C

24

30

L

M

N

16

MN : BC = 18 : 24 = 3 : 4

perimeter : PERIMETER = 3 : 4

210

18

= 4(perimeter)

= 16 + 24 + 30 = 70

Let’s ReviewCore Lesson

6 in

30 in

4 in

perimeter

  =

P = 150 in

small : LARGE = 4 : 30

2P = 300

= 2 : 15

= 2(6) + 2(4) = 20 in

How large is the frame?

In this lesson you have learned how to compute an

actual perimeter from a scale drawing by using ratios and proportions.

Let’s ReviewGuided Practice

Find the base of the large triangle if the scale factor of the small perimeter to the large

perimeter is 3 : 5 for these similar triangles.

6

15

6

Let’s ReviewExtension Activities

If the scale factor of a small figure to a large figure is 3 to 7, what will be the scale factor of the large figure’s perimeter to the small figure’s perimeter? Explain.

Let’s ReviewExtension Activities

Find the length of side DF.

D

E

F

P

Q

R? cm

15 cm

Perimeter of DEF = 90 cm

7 cm

Perimeter of PQR = 40 cm

Let’s ReviewQuick Quiz

1. Find the perimeter of the large rectangle.

84

28

16

Let’s ReviewQuick Quiz

2. If the scale factor of the large perimeter to the small perimeter is 9:5, find the length of AD.

A

B C

D30

16 12

17L M

NK