1. 12-8 Congruent and Similar Solids
You compared surface areas and volumes of spheres.
Identify congruent or similar solids.
Use properties of similar solids.

2. Similar Cylinders Are the cylinders similar?
What is their similarity ratio?
Find the volume of each cylinder.
What is the ratio of their volumes? 3 9
V = π(3)2(9)
V = 81π 1 3
V = π(1)23
V = 3π

3. Similar solids have exactly the same shape, but not necessarily the same size.
All spheres are similar and all cube are similar.
In similar solids, the corresponding linear measures (like height or radius) have equal ratios.
The common ratio is called the scale factor.
p. 896

4. Congruent solids have exactly the same shape and the same size.
Congruent solids are similar solids that have a scale factor of 1:1
p. 896

5. Similar Animals
The weight of an object of a given density is proportional to its volume. Suppose the measurements of one animal are four times the measurements of another, and the two animals are geometrically similar. How many times greater is the weight of the larger animal than the weight of the smaller?
Similarity ratio 4/1
V=(4)3=64

6. Two years ago, Josh bought a puppy that weighed 2 lb
Two years ago, Josh bought a puppy that weighed 2 lb. Since then, it has tripled in length. About how much does the dog weigh now?
The similarity ratio is 3/1.
Ratio of the volumes would be (3/1)3= 27
Weight is proportional to volume, so the dog now weighs about 2 ×27 = 54 lbs

7. What is the ratio of the volumes of the two cubes shown?
20 cm
40 cm

8. Compare the ratios between the corresponding parts of the cones.
B. Determine whether the pair of solids is similar, congruent, or neither. If the solids are similar, state the scale factor.
Compare the ratios between the corresponding parts of the cones.
Substitution =1
Simplify.
Substitution
Answer: Since the ratios are not the same, the cones are neither similar nor congruent.

9. A. Determine whether the pair of solids is similar, congruent, or neither.
A. similar
B. congruent
C. neither

10. p. 897

11. Two similar cones have radii of 9 inches and 12 inches
Two similar cones have radii of 9 inches and 12 inches. What is the ratio of the volume of the smaller cone to the volume of the larger cone?
First, find the scale factor.
Write a ratio comparing the radii.
If the scale factor is , then the ratio of the volumes is
Answer: So, the ratio of the volume is 27:64.

12. Two similar cones have radii of 5 inches and 15 inches
Two similar cones have radii of 5 inches and 15 inches. What is the ratio of the volume of the smaller cone to the volume of the larger cone?
A. 1:3
B. 1:9
C. 1:27
D. 1:81

13. SOFTBALLS The softballs shown are similar spheres
SOFTBALLS The softballs shown are similar spheres. Find the radius of the smaller softball if the radius of the larger one is about 1.9 cubic inches.
Understand You know the volume of the softballs.
Plan Use Theorem 12.1 to write a ratio comparing the volumes. Then find the scale factor and use it to find r.
Solve
Write a ratio comparing volumes. =
Simplify. ≈
Write as

14. Ratio of radii
Scale factor
Find the cross products.
r ≈ 1.45
Solve for r.
Answer: So, the radius of the smaller softball is about 1.45 inches.

15. The ratio of the volumes of two similar cylinders is 125/27
The ratio of the volumes of two similar cylinders is 125/27. What is the ratio of their radii? ?
Vol ratio ?
Sim ratio

16. CONTAINERS The containers below are similar cylinders
CONTAINERS The containers below are similar cylinders. Find the height h of the smaller container.
A. 2 in.
B. 3 in.
C. 4 in.
D. 5 in.

17. Similarity Ratios Similarity Ratio SurfaceArea Ratio Volume Ratio
(any part)
SurfaceArea Ratio
Volume Ratio

18. 12-8 Assignment
Page 899, 6-13