### AA Similarity Postulate

By definition, two triangles are similar if all their corresponding angles are congruent and their corresponding sides are proportional. It is not necessary to check all angles and sides in order to tell if two triangles are similar. In fact, if you only know that two pairs of corresponding angles are congruent that is enough information to know that the triangles are similar. This is called the **AA Similarity Postulate.**

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

If \begin{align*}\angle A \cong \angle Y\end{align*} and \begin{align*}\angle B \cong \angle Z\end{align*}, then \begin{align*}\triangle ABC \sim \triangle YZX\end{align*}.

What if you were given a pair of triangles and the angle measures for two of their angles? How could you use this information to determine if the two triangles are similar?

### Examples

#### Example 1

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

Yes, there are three similar triangles that each have a right angle. \begin{align*}DGE \sim FGD \sim FDE\end{align*}.

#### Example 2

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

By the reflexive property, \begin{align*}\angle H \cong \angle H\end{align*}. Because the horizontal lines are parallel, \begin{align*}\angle L \cong \angle K\end{align*} (corresponding angles). So yes, there is a pair of similar triangles. \begin{align*} HLI \sim HKJ\end{align*}.

#### Example 3

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

Compare the angles to see if we can use the AA Similarity Postulate. Using the Triangle Sum Theorem, \begin{align*}m \angle G = 48^{\circ}\end{align*} and \begin{align*}m \angle M = 30^\circ\end{align*} So, \begin{align*}\angle F \cong \angle M, \angle E \cong \angle L\end{align*} and \begin{align*}\angle G \cong \angle N\end{align*} and the triangles are similar. \begin{align*}\triangle FEG \sim \triangle MLN\end{align*}.

#### Example 4

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

Compare the angles to see if we can use the AA Similarity Postulate. Using the Triangle Sum Theorem, \begin{align*}m \angle C = 39^{\circ}\end{align*} and \begin{align*}m \angle F = 59^{\circ}\end{align*}. \begin{align*}m \angle C \neq m \angle F\end{align*}, So \begin{align*}\triangle ABC\end{align*} and \begin{align*}\triangle DEF\end{align*} are not similar.

#### Example 5

\begin{align*}\triangle LEG \sim \triangle MAR\end{align*} by AA. Find \begin{align*}GE\end{align*} and \begin{align*}MR\end{align*}.

Set up a proportion to find the missing sides.

\begin{align*}\frac{24}{32} &= \frac{MR}{20} && \qquad \ \frac{24}{32} = \frac{21}{GE}\\ 480 &= 32MR && \quad 24GE = 672\\ 15 &= MR && \qquad GE = 28\end{align*}

When two triangles are similar, the corresponding sides are proportional. But, what are the corresponding sides? Using the triangles from this example, we see how the sides line up in the diagram to the right.

### Review

Use the diagram to complete each statement.

- \begin{align*}\triangle SAM \sim \triangle\end{align*} ______
- \begin{align*}\frac{SA}{?} = \frac{SM}{?} = \frac{?}{RI}\end{align*}
- \begin{align*}SM\end{align*} = ______
- \begin{align*}TR\end{align*} = ______
- \begin{align*}\frac{9}{?} = \frac{?}{8}\end{align*}

Answer questions 6-9 about trapezoid \begin{align*}ABCD\end{align*}.

- 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 \begin{align*}AB = 10, AE = 7,\end{align*} and \begin{align*}DC = 22\end{align*}, find \begin{align*}AC\end{align*}. Be careful!

Use the triangles to the left for questions 10-14.

\begin{align*}AB = 20, DE = 15\end{align*}, and \begin{align*}BC = k\end{align*}.

- Are the two triangles similar? How do you know?
- Write an expression for \begin{align*}FE\end{align*} in terms of \begin{align*}k\end{align*}.
- If \begin{align*}FE = 12,\end{align*}, what is \begin{align*}k\end{align*}?
- 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 _______.
How do congruent triangles and similar triangles differ? How are they the same?*Writing*

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

### Review (Answers)

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

### Resources