1. Sec Math II Unit 8 Lesson 3 Class Notes
Similarity of Triangles
Sec Math II
Unit 8 Lesson 3
Class Notes
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2. In geometry, two polygons are similar when one is a replica (scale model) of the other.
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3. Consider Dr. Evil and Mini Me from Mike Meyers’ hit movie Austin Powers. Mini Me is supposed to be an exact replica of Dr. Evil.
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5. The following are similar figures.II
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6. The following are non-similar figures.II
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7. Feefee the mother cat, lost her daughters, would you please help her to find her daughters. Her daughters have the similar footprint with their mother.
Feefee’s footprint
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8. Which of the following is similar to the above triangle?1.
Which of the following is similar to the above triangle? B A C
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9. Similar triangles are triangles that have the same shape but not necessarily the same size.D F E
ABC  DEF
When we say that triangles are similar there are several repercussions that come from it.
A  D AB DE BC EF AC DF
= =
B  E
C  F

10. Six of those statements are true as a result of the similarity of the two triangles. However, if we need to prove that a pair of triangles are similar how many of those statements do we need? Because we are working with triangles and the measure of the angles and sides are dependent on each other. We do not need all six. There are three special combinations that we can use to prove similarity of triangles.
1. PPP Similarity Theorem
 3 pairs of proportional sides
2. PAP Similarity Theorem
 2 pairs of proportional sides and congruent angles between them
3. AA Similarity Theorem
 2 pairs of congruent angles

11. ABC  DFE E F D 1. PPP Similarity Theorem
 3 pairs of proportional sides
9.6
10.4 A B C 5 13 12 4
ABC  DFE

12. GHI  LKJ mH = mK 2. PAP Similarity Theorem
 2 pairs of proportional sides and congruent angles between them L J K
7.5 G H I 5
70
70 7
10.5
mH = mK
GHI  LKJ

13. The PAP Similarity Theorem does not work unless the congruent angles fall between the proportional sides. For example, if we have the situation that is shown in the diagram below, we cannot state that the triangles are similar. We do not have the information that we need. L J K
7.5 G H I 5
50 7
50
10.5
Angles I and J do not fall in between sides GH and HI and sides LK and KJ respectively.

14. MNO  QRP mN = mR mO = mP 3. AA Similarity Theorem
 2 pairs of congruent angles Q P R M N O
70
50
50
70
mN = mR
MNO  QRP
mO = mP

15. TSU  XZY mT = mX mS = mZ
It is possible for two triangles to be similar when they have 2 pairs of angles given but only one of those given pairs are congruent. S T U X Y Z
34
34
34
34
59
59
mT = mX
87
59
mS = mZ
mS = 180- (34 + 87)
TSU  XZY
mS = 180- 121
mS = 59

16. Note: One triangle is a scale model of the other triangle.
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17. How do we know if two triangles are similar or proportional?
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18. Triangles are similar (~) if corresponding angles are equal and the ratios of the lengths of corresponding sides are equal.
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19. The sum of the measure of the angles of a triangle is 1800.
Interior Angles of Triangles A B C
The sum of the measure of the angles of a triangle is 1800.
Ð A + Ð B + ÐC =1800
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20. Determine whether the pair of triangles is similar. Justify your answer.
Answer: Since the corresponding angles have equal measures, the triangles are similar.
Example 6-1b

21. If the product of the extremes equals the product of the means then a proportion exists.
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22. This tells us that  ABC and  XYZ are similar and proportional.
This tells us that  ABC and  XYZ are similar and proportional.
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23. Q: Can these triangles be similar?
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24. Answer—Yes, right triangles can also be similar but use the criteria.
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26. Do we have equality?
This tells us our triangles are not similar. You can’t have two different scaling factors!
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27. If we are given that two triangles are similar or proportional what can we determine about the triangles?
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28. The two triangles below are known to be similar, determine the missing value X.
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30. In the figure, the two triangles are similar. What are c and d ?B C P Q R 10 6 c 5 4 d
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31. In the figure, the two triangles are similar. What are c and d ?B C P Q R 10 6 c 5 4 d
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32. Sometimes we need to measure a distance indirectly
Sometimes we need to measure a distance indirectly. A common method of indirect measurement is the use of similar triangles. h 6 17
102
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33. Two triangles are called “similar”
if their corresponding angles have
the same measure.      

34. a A b B c C Two triangles are called “similar”
if their corresponding angles have
the same measure. 
Ratios of corresponding
sides are equal. C A  a c     b B a A b B c C = =

35. Mary is 5 ft 6 inches tall.
She casts a 2 foot shadow.
The tree casts a 7 foot shadow.
How tall is the tree?

36. Mary is 5 ft 6 inches tall.
She casts a 2 foot shadow.
The tree casts a 7 foot shadow.
How tall is the tree?
Mary’s height
Tree’s height
Mary’s shadow
Tree’s shadow = x
5.5 2 7

37. Mary is 5 ft 6 inches tall.
She casts a 2 foot shadow.
The tree casts a 7 foot shadow.
How tall is the tree?
5.5 x 2 7 =
Mary’s height
Tree’s height
Mary’s shadow
Tree’s shadow = x
5.5 2 7

38. 5.5 x 2 7 =
7 ( 5.5 ) = 2 x
= 2 x
x =
The height of the tree is feet

39. Congruent Figures
In order to be congruent, two figures must be the same size and same shape.

40. Similar Figures
Similar figures must be the same shape, but their sizes may be different.

41. Similar Figures
This is the symbol that means “similar.”
These figures are the same shape but different sizes.

42. SIZES
Although the size of the two shapes can be different, the sizes of the two shapes must differ by a factor. 4 2 6 6 3 3 1 2

43. In this case, the factor is x 2.
SIZES
In this case, the factor is x 2. 4 2 6 6 3 3 2 1

44. Or you can think of the factor as 2.
SIZES
Or you can think of the factor as 4 2 6 6 3 3 2 1

45. When you have a photograph enlarged, you make a similar photograph.
Enlargements
When you have a photograph enlarged, you make a similar photograph.
X 3

46. Reductions
A photograph can also be shrunk to produce a slide. 4

47. Determine the length of the unknown side.15 12 ? 4 3 9

48. These triangles differ by a factor of 3.
= 5 15 12 ? 4 3 9

49. Determine the length of the unknown side.? 2 24 4

50. These dodecagons differ by a factor of 6.?
2 x 6 = 12 2 24 4

51. Sometimes the factor between 2 figures is not obvious and some calculations are necessary.15 12 8 10 18 12
? =

52. To find this missing factor, divide 18 by 12.15 12 8 10 18 12
? =

53. 18 divided by 12 = 1.5

54. The value of the missing factor is 1.5.15 12 8 10 18 12
1.5 =

55. When changing the size of a figure, will the angles of the figure also change?40 70 ? ? 70

56. Nope! Remember, the sum of all 3 angles in a triangle MUST add to 180 degrees. If the size of the angles were increased, the sum would exceed 180 degrees. 40 40 70 70 70 70

57. We can verify this fact by placing the smaller triangle inside the larger triangle.40 40 70 70 70 70

58. The 40 degree angles are congruent.70 70 70 70

59. The 70 degree angles are congruent.40 40 70 70 70 70 70

60. The other 70 degree angles are congruent.4 40 70 70 70 70 70

61. Find the length of the missing side.50 ? 30 6 40 8

62. This looks messy. Let’s translate the two triangles.50 ? 30 6 40 8

63. Now “things” are easier to see.50 30 ? 6 40 8

64. The common factor between these triangles is 5.50 30 ? 6 40 8

65. So the length of the missing side is…?

66. That’s right! It’s ten! 50 30 10 6 40 8

67. Similarity is used to answer real life questions.
Suppose that you wanted to find the height of this tree.

68. Unfortunately all that you have is a tape measure, and you are too short to reach the top of the tree.

69. You can measure the length of the tree’s shadow.
10 feet

70. Then, measure the length of your shadow.
10 feet
2 feet

71. If you know how tall you are, then you can determine how tall the tree is.
6 ft
10 feet
2 feet

72. The tree must be 30 ft tall. Boy, that’s a tall tree!
10 feet
2 feet

73. Similar figures “work” just like equivalent fractions.30 5 11 66

74. These numerators and denominators differ by a factor of 6.30 6 5 6 11 66

75. Two equivalent fractions are called a proportion.30 5 11 66

76. Similar Figures
So, similar figures are two figures that are the same shape and whose sides are proportional.

77. Practice Time!

78. 1) Determine the missing side of the triangle.? 9 5  3 4 12

79. 1) Determine the missing side of the triangle.15 9 5  3 4 12

80. 2) Determine the missing side of the triangle.36 36 6 6  4 ?

81. 2) Determine the missing side of the triangle.36 36 6 6  4 24

82. 3) Determine the missing sides of the triangle.39 ?  33 ? 8 24

83. 3) Determine the missing sides of the triangle.39 13  33 11 8 24

84. 4) Determine the height of the lighthouse. ? 8
2.5 10

85. 4) Determine the height of the lighthouse. 32 8
2.5 10

86. 5) Determine the height of the car. ? 3 5 12

87. 5) Determine the height of the car.
7.2  3 5 12