Big Ideas Math Geometry, 2014
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Big Ideas Math Geometry, 2014 View details
2. Proving Triangle Similarity by AA
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Exercise 26 Page 432

What are the values of m∠ L and m∠ X?

No, see solution.

Practice makes perfect

Let's recall the Angle-Angle (AA) Similarity Theorem.

AA Similarity Theorem

If two angles of one triangle are congruent to two angles of another triangle, then the two triangles are similar.

Note that if two angles of one triangle are congruent to two angles of another triangle, the third angle of the first triangle is also congruent to the third angle of the second triangle. Conversely, if two triangles are similar, all corresponding angles are congruent. Therefore, we can claim the following.

Two triangles are similar if and only if all angles of one triangle are congruent to corresponding angles of another triangle.

Let's check if it is possible for △ JKL and △ XYZ to have three pairs of congruent angles. We are given the sum of the measures of ∠ J and ∠ K, and the sum of the measures of ∠ Y and ∠ Z. m∠ J +m∠ K=85^(∘) and m∠ Y+m∠ Z=80^(∘) Using the Triangle Sum Theorem we can find m∠ L and m∠ X. m∠ L=180^(∘)-85^(∘)=95^(∘) and m∠ X=180^(∘)-80^(∘)=100^(∘) Since m∠ L>80^(∘), triangle △ XYZ does not have an angle congruent to ∠ L. Therefore, △ JKL and △ XYZ cannot have three pairs of congruent angles. It is not possible for these triangles to be similar.