### AA Similarity

The Third Angle Theorem states if two angles are congruent to two angles in another triangle, the third angles are congruent too. Because a triangle has

#### Investigation: Constructing Similar Triangles

Tools Needed: pencil, paper, protractor, ruler

- Draw a
45∘ angle. Extend the horizontal side and then draw a60∘ angle on the other side of this side. Extend the other side of the45∘ angle and the60∘ angle so that they intersect to form a triangle. What is the measure of the third angle? Measure the length of each side. - Repeat Step 1 and make the horizontal side between the
45∘ and60∘ angle at least 1 inch longer than in Step 1. This will make the entire triangle larger. Find the measure of the third angle and measure the length of each side. Find the ratio of the sides. Put the sides opposite the45∘ angles over each other, the sides opposite the60∘ angles over each other, and the sides opposite the third angles over each other. What happens?

*Watch this video beginning at the 2:09 mark.*

**AA Similarity Postulate:** If two angles in one triangle are congruent to two angles in another triangle, the two triangles are similar.

The AA Similarity Postulate is a shortcut for showing that two ** triangles** are similar. If you know that two angles in one triangle are congruent to two angles in another, which is now enough information to show that the two triangles are similar. Then, you can use the similarity to find the lengths of the sides.

#### Determining if Two Triangles are Similar

1. Determine if the following two triangles are similar. If so, write the similarity statement.

Find the measure of the third angle in each triangle.

2. Determine if the following two triangles are similar. If so, write the similarity statement.

3. Are the following triangles similar? If so, write the similarity statement.

Because

### Examples

Are the following triangles similar? If so, write a similarity statement.

#### Example 1

Yes,

#### Example 2

Yes,

#### Example 3

No, though

### Review

Use the diagram to complete each statement.

Answer questions 6-9 about trapezoid

Name two similar triangles. How do you know they are similar?

Write a true proportion.

Name two other triangles that might *not* be similar.

If

** Writing** How many angles need to be congruent to show that two triangles are similar? Why?

** Writing** How do congruent triangles and similar triangles differ? How are they the same?

Use the triangles below for questions 12-15.

Are the two triangles similar? How do you know?

Write an expression for

If

Fill in the blanks: If an acute angle of a _______ triangle is congruent to an acute angle in another ________ triangle, then the two triangles are _______.

Use the diagram below to answer questions 16-20.

Draw the three separate triangles in the diagram.

Explain why

Complete the following proportionality statements.

### Review (Answers)

To view the Review answers, open this PDF file and look for section 7.4.