McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
5. Parts of Similar Triangles
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Exercise 27 Page 589

Use the definition of angle bisector and the Angle-Angle (AA) Similarity Postulate.

Statements
Reasons
1.
△ QTS ~ △ XWZ, TR and WY are angle bisectors
1.
Given
2.
m∠ RTQ = 1/2m∠ T
m∠ YWX = 1/2m∠ W
2.
Definition of angle bisector
3.
∠ T ≅ ∠ W
3.
Definition of similar triangles
4.
m∠ T = m∠ W
4.
Definition of congruent angles
5.
m∠ RTQ = 1/2m∠ T
m∠ YWX = 1/2m∠ T
5.
Substitution
6.
m∠ RTQ = m∠ YWX
6.
Substitution
7.
∠ RTQ ≅ ∠ YWX
7.
Definition of congruent angles
8.
∠ Q ≅ ∠ X
8.
Definition of similar triangles
9.
△ TRQ ~ △ WYX
9.
AA Similarity Postulate
10.
TR/WY = QT/XW
10.
Definition of similar triangles
Practice makes perfect

Let △ QTS and △ XWZ be a pair of triangles such that they are similar. Also, let TR and WY be angle bisectors.

Because corresponding angles of similar triangles are congruent, we have the following pair of congruences. ∠ T≅ ∠ W and ∠ Q ≅ ∠ X ⇓ m∠ T = m∠ W and m∠ Q = m∠ XBy the definition of an angle bisector we have that m∠ RTQ = 12m∠ T and m∠ YWX = 12m∠ W.
m∠ T = m∠ W
1/2m∠ T = 1/2m∠ W
m∠ RTQ = m∠ YWX
From the above, ∠ RTQ ≅ ∠ YWX.

Then, △ TRQ ~ △ WYX because of the Angle-Angle (AA) Similarity Postulate. This leads us to set the required proportion. TR/WY = QT/XW ✓

Two-Column Proof

Given: & △ QTS ~ △ XWZ, TR and WY & are angle bisectors Prove: & TRWY = QTXW We will summarize the proof we did above in the following two-column table.

Statements
Reasons
1.
△ QTS ~ △ XWZ, TR and WY are angle bisectors
1.
Given
2.
m∠ RTQ = 1/2m∠ T
m∠ YWX = 1/2m∠ W
2.
Definition of angle bisector
3.
∠ T ≅ ∠ W
3.
Definition of similar triangles
4.
m∠ T = m∠ W
4.
Definition of congruent angles
5.
m∠ RTQ = 1/2m∠ T
m∠ YWX = 1/2m∠ T
5.
Substitution
6.
m∠ RTQ = m∠ YWX
6.
Substitution
7.
∠ RTQ ≅ ∠ YWX
7.
Definition of congruent angles
8.
∠ Q ≅ ∠ X
8.
Definition of similar triangles
9.
△ TRQ ~ △ WYX
9.
AA Similarity Postulate
10.
TR/WY = QT/XW
10.
Definition of similar triangles