1. Ch 9.3
Determine whether the triangles are similar.
Yes, 5/3 = 12/7.2 = 13/7.8
The quadrilaterals are similar. Find the scale factor of the larger quadrilateral to the smaller quadrilateral.
3:2
The triangles are similar. Find x and y.
x = 8.5 y = 9.5 __
Two pentagons are similar with a scale factor of The perimeter of the larger pentagon is 42 feet. What is the perimeter of the smaller pentagon? 3 7 18

2. Students prove basic theorems involving similarity.
Ch 9.3
Ch 9.3 Similar Triangles
Learning Target:
I will be able to identify similar triangles using the AA Similarity Postulate and the SSS and SAS Similarity Theorems and use similar triangles to solve problems.
Standard 4.0
Students prove basic theorems involving similarity.

3. Ch 9.3
Postulate 9-1
Concept

4. Ch 9.3
Use the AA Similarity Postulate
A. Determine whether the triangles are similar. If so, write a similarity statement. Explain your reasoning.
Since mB = mD, B D.
By the Triangle Sum Theorem, mA = 180, so mA = 80.
Since mE = 80, A E.
Answer: So, ΔABC ~ ΔEDF by the AA Similarity.
Example 1

5. Ch 9.3
Use the AA Similarity Postulate
B. Determine whether the triangles are similar. If so, write a similarity statement. Explain your reasoning.
QXP NXM by the Vertical Angles Theorem.
Since QP || MN, Q N.
Answer: So, ΔQXP ~ ΔNXM by AA Similarity.
Example 1

6. Ch 9.3
A. Determine whether the triangles are similar. If so, write a similarity statement.
A. Yes; ΔABC ~ ΔFGH
B. Yes; ΔABC ~ ΔGFH
C. Yes; ΔABC ~ ΔHFG
D. No; the triangles are not similar.
Example 1

7. Ch 9.3
B. Determine whether the triangles are similar. If so, write a similarity statement.
A. Yes; ΔWVZ ~ ΔYVX
B. Yes; ΔWVZ ~ ΔXVY
C. Yes; ΔWVZ ~ ΔXYV
D. No; the triangles are not similar.
Example 1

8. Ch 9.3
9-2
9-3
Concept

9. Ch 9.3
Theorem 9-2
Concept

10. Ch 9.3
Use the SSS and SAS Similarity Theorems
A. Determine whether the triangles are similar. If so, write a similarity statement. Explain your reasoning.
Answer: So, ΔABC ~ ΔDEC by the SSS Similarity Theorem.
Example 2

11. Ch 9.3
Use the SSS and SAS Similarity Theorems
B. Determine whether the triangles are similar. If so, write a similarity statement. Explain your reasoning.
By the Reflexive Property, M  M.
Answer: Since the lengths of the sides that include M are proportional, ΔMNP ~ ΔMRS by the SAS Similarity Theorem.
Example 2

12. Ch 9.3
A. Determine whether the triangles are similar. If so, choose the correct similarity statement to match the given data.
A. ΔPQR ~ ΔSTR by SSS Similarity Theorem
B. ΔPQR ~ ΔSTR by SAS Similarity Theorem
C. ΔPQR ~ ΔSTR by AA Similarity Theorem
D. The triangles are not similar.
Example 2

13. Ch 9.3
B. Determine whether the triangles are similar. If so, choose the correct similarity statement to match the given data.
A. ΔAFE ~ ΔABC by SAS Similarity Theorem
B. ΔAFE ~ ΔABC by SSS Similarity Theorem
C. ΔAFE ~ ΔACB by SAS Similarity Theorem
D. ΔAFE ~ ΔACB by SSS Similarity Theorem
Example 2

14. Ch 9.3
If ΔRST and ΔXYZ are two triangles such that = , which of the following would be sufficient to prove that the triangles are similar? A B C R  S D RS 2 XY 3
___
Example 3

15. Ch 9.3
If ΔRST and ΔXYZ are two triangles such that = , which of the following would be sufficient to prove that the triangles are similar? RS 2 XY 3
___
Read the Test Item
You are given that = and asked to identify which additional information would be sufficient to prove that ΔRST ~ ΔXYZ. __ 2 3
___ RS XY
Example 3

16. Ch 9.3
If ΔRST and ΔXYZ are two triangles such that = , which of the following would be sufficient to prove that the triangles are similar? RS 2 XY 3
___ __ 2 3
Solve the Test Item
Since = , you know that these two sides are proportional with a scale factor of Check each answer choice until you find one that supplies sufficient information to prove that ΔRST ~ ΔXYZ.
___ RS XY
Example 3

17. Ch 9.3
A B C R  S D
Choice A
If = , then you know that the other two sides are proportional. You do not, however, know whether the scale factor is , as determined by Therefore, this is not sufficient information.
___ RT XZ
___ ST YZ __ 2 3
___ RS XY
Example 3

18. Ch 9.3
A B C R  S D __ 2 3
Choice B
If = = , then you know that all the sides are proportional with the same scale factor, This is sufficient information by the SSS Similarity Theorem to determine that the triangles are similar.
___ RS XY RT XZ
Answer: B
Example 3

19. Ch 9.3 A. = B. mA = 2mD C. = D. = AC DC 4 3 BC 5 EC
Given ΔABC and ΔDEC, which of the following would be sufficient information to prove the triangles are similar?
A =
B. mA = 2mD
C =
D =
___ AC DC __ 4 3 BC 5 EC
Example 3

20. Ch 9.3 ALGEBRA Given , RS = 4, RQ = x + 3, QT = 2x + 10, UT = 10,
Parts of Similar Triangles
ALGEBRA Given , RS = 4,
RQ = x + 3, QT = 2x + 10, UT = 10,
find RQ and QT.
Since because they are alternate interior angles. By AA Similarity, ΔRSQ ~ ΔTUQ. Using the definition of similar polygons,
Example 4

21. Ch 9.3 ALGEBRA Given , RS = 4, RQ = x + 3, QT = 2x + 10, UT = 10,
Parts of Similar Triangles
ALGEBRA Given , RS = 4,
RQ = x + 3, QT = 2x + 10, UT = 10,
find RQ and QT.
Substitution
Cross Products Property
Distributive Property
Subtract 8x and 30 from each side.
Divide each side by 2.
Now find RQ and QT.
Example 4

22. Ch 9.3 ALGEBRA Given , RS = 4, RQ = x + 3, QT = 2x + 10, UT = 10,
Parts of Similar Triangles
ALGEBRA Given , RS = 4,
RQ = x + 3, QT = 2x + 10, UT = 10,
find RQ and QT.
Now find RQ and QT.
Answer: RQ = 8; QT = 20
Example 4

23. Ch 9.3
ALGEBRA Given AB = 38.5, DE = 11, AC = 3x + 8, and CE = x + 2, find AC.
A. 2
B. 4
C. 12
D. 14
Example 4

24. Ch 9.3
Indirect Measurement
SKYSCRAPERS Josh wanted to measure the height of the Sears Tower in Chicago. He used a 12-foot light pole and measured its shadow at 1 p.m. The length of the shadow was 2 feet. Then he measured the length of Sears Tower’s shadow and it was 242 feet at the same time. What is the height of the Sears Tower?
Understand Make a sketch of the situation.
Example 5

25. Ch 9.3
Plan In shadow problems, you can assume that the angles formed by the Sun’s rays with any two objects are congruent and that the two objects form the sides of two right triangles. Since two pairs of angles are congruent, the right triangles are similar by the AA Similarity Postulate.
So the following proportion can be written.
Example 5

26. Ch 9.3
Solve Substitute the known values and let x be the height of the Sears Tower.
Substitution
Cross Products Property
Simplify.
Divide each side by 2.
Answer: The Sears Tower is 1452 feet tall.
Example 5

27. Ch 9.3
LIGHTHOUSES On her trip along the East coast, Jennie stops to look at the tallest lighthouse in the U.S. located at Cape Hatteras, North Carolina. At that particular time of day, Jennie measures her shadow to be 1 foot 6 inches in length and the length of the shadow of the lighthouse to be 53 feet 6 inches. Jennie knows that her height is 5 feet 6 inches. What is the height of the Cape Hatteras lighthouse to the nearest foot?
A. 196 ft B ft
C. 441 ft D ft
Example 5

28. Ch 9.3
Concept