Michael is 6 feet tall and is standing outside next to his younger sister. He notices that he can see both of their shadows and decides to measure each shadow. His shadow is 8 feet long and his sister's shadow is 5 feet long. How tall is Michael's sister?

### Applications of Similar Triangles

If two triangles are similar, then their corresponding angles are congruent and their corresponding sides are proportional. There are three criteria for proving that triangles are similar:

**AA:**If two triangles have two pairs of congruent angles, then the triangles are similar.**SAS:**If two sides of one triangle are proportional to two sides of another triangle and their included angles are congruent, then the triangles are similar.**SSS:**If three sides of one triangle are proportional to three sides of another triangle, then the triangles are similar.

Once you know that two triangles are similar, you can use the fact that their corresponding sides are proportional and their corresponding angles are congruent to solve problems.

Let's take a look at some example problems.

1. Prove that the two triangles below are similar.

The triangles are similar by

Use the Pythagorean Theorem to find

Use the fact that the triangles are similar to find the missing sides of

ABDE=3→AB3=3→AB=9 BCEF=3→BC33√=3→BC=93√

#### The triangles are called 30-60-90 triangles because of their angles measures.

Explain why all 30-60-90 triangles are similar.

All 30-60-90 triangles are similar by

2. Use the triangles from #1 to find the ratios between the three sides of any 30-60-90 triangle.

3. Find the missing sides of the triangle below.

The side opposite the

4. Create similar triangles in order to solve for

Extend

Next, solve for

**Examples**

**Example 1**

Earlier, you were asked how tall Michael's sister is. You can answer this question using applications of similar triangles.

The sun creates shadows at the same angle for both Michael and his sister. Assuming they are both standing up straight and making right angles with the ground, similar triangles are created.

Corresponding sides are proportional because the triangles are similar.

Cross multiply and solve for his sister's height. His sister is 3.75 feet tall.

#### Example 2

Prove that all isosceles right triangles are similar.

Consider two generic isosceles right triangles:

Two pairs of sides are proportional with a ratio of

#### Example 3

Find the measures of the angles of an isosceles right triangle. Why are isosceles right triangles called 45-45-90 triangles?

The base angles of an isosceles triangle are congruent. If the vertex angle is

#### Example 4

Use the Pythagorean Theorem to find the missing side of an isosceles right triangle whose legs are each length

The missing side is the hypotenuse of the right triangle,

#### Example 5

Use what you have learned in #1-#3 to find the missing sides of the right triangle below without using the Pythagorean Theorem.

If one of the legs is 3, then the other leg is also 3, so

### Review

1. Explain why all 30-60-90 triangles are similar.

2. The ratio between the sides of any 30-60-90 triangle is ________:________:________.

Find the missing sides of each triangle:

3.

4.

5.

6. Explain why all 45-45-90 triangles are similar.

7. The ratio between the sides of any 45-45-90 triangle is ________:________:________.

Find the missing sides of each triangle:

8.

9.

10.

Use the figure below for #11 and #12.

11. Prove that \begin{align*}\Delta ABC \sim \Delta EFD\end{align*}.

12. Find the value of \begin{align*}x\end{align*}.

Use the figure below for #13-#15.

13. Find \begin{align*}m \angle ADB\end{align*}. What type of triangle is \begin{align*}\Delta ADB\end{align*}?

14. Find \begin{align*}BD\end{align*} and \begin{align*}AB\end{align*}.

15. Find \begin{align*}AC\end{align*}.

### Review (Answers)

To see the Review answers, open this PDF file and look for section 6.7.