1. Warm Up
Find the area of each figure. Give exact answers, using  if necessary.
1. a square in which s = 4 m
2. a circle in which r = 2 ft
3. What’s bigger: two 10in pizzas or a 20in pizza? Are they the same?
16 m2
4 ft2

2. Lesson 9-5 Effects of Changing Dimensions

3. Rectangle A Rectangle B 2” 4” 3” 6” P = P = A = A =
Find the perimeter and area of each rectangle. Compare the larger rectangle to the smaller using division.
Rectangle A Rectangle B
2” ”
3” ”
P = P =
A = A =

4. What if we only change length and leave width alone?
Rectangle A: 2x Rectangle B: 6X4
What was the scale factor for the length change?
How did this affect the Area?

5. What if we change length and width but by different factors?
Rectangle A:2x Rectangle B:6x8
Length change factor:
Width change factor:
Area change:

6. Is this true for all shapes or just rectangles?
Consider circles
Circle A: r = Circle B: r = 9
Scale Factor for radius?
Area change?

7. What about Squares? Square A: s=4 Square B: s = 12 Scale factor?
Area change?

8. When the dimensions of a figure are changed proportionally, the figure will be similar to the original figure.

9. Example 3A: Effects of Changing Area
A circle has a circumference of 32 in. If the area is multiplied by 4, what happens to the radius?
Helpful Hint
Helpful Hint

10. Example 3B: Effects of Changing Area
An equilateral triangle has a perimeter of 21m. If the area is multiplied by , what happens to the side length?

11. Summary
The relationship between perimeter and area of similar figures is as follows:
If you know the change in perimeter, the area is changed by that number squared.
If you know the change in area you can square root to find the change in perimeter.
Homework: p ,30-32,41,42
Quiz tomorrow!
Test on Thursday!