# 8.4: Special Right Triangles

**At Grade**Created by: CK-12

## Learning Objectives

- Learn and use the 45-45-90 triangle ratio.
- Learn and use the 30-60-90 triangle ratio.

## Review Queue

Find the value of the missing variables. Simplify all radicals.

- Is 9, 12, and 15 a right triangle?
- Is 3,
33√ , and 6 a right triangle?

**Know What?** A baseball diamond is a square with sides that are 90 feet long. Each base is a corner of the square. What is the length between

## Isosceles Right Triangles

There are two special right triangles. The first is an isosceles right triangle.

**Isosceles Right Triangle:** A right triangle with congruent legs and acute angles. This triangle is also called a 45-45-90 triangle (after the angle measures).

**Investigation 8-2: Properties of an Isosceles Right Triangle**

Tools Needed: Pencil, paper, compass, ruler, protractor

1. Draw an isosceles right triangle with 2 inch legs and the

2. Find the measure of the hypotenuse, using the Pythagorean Theorem. Simplify the radical.

What do you notice about the length of the legs and hypotenuse?

3. Now, let’s say the legs are of length

**45-45-90 Theorem:** If a right triangle is isosceles, then its sides are

For any isosceles right triangle, the legs are *the hypotenuse is always**all isosceles right triangles are similar.*

**Example 1:** Find the length of the missing sides.

a)

b)

**Solution:** Use the

a)

b)

**Example 2:** Find the length of

a)

b)

**Solution:** Use the

a)

b) Here, we are given the hypotenuse. Solve for

In part b, we ** rationalized the denominator** which we learned in the first section.

## 30-60-90 Triangles

The second special right triangle is called a 30-60-90 triangle, after the three angles. To draw a 30-60-90 triangle, start with an equilateral triangle.

**Investigation 8-3: Properties of a 30-60-90 Triangle**

Tools Needed: Pencil, paper, ruler, compass

1. Construct an equilateral triangle with 2 inch sides.

http://www.mathsisfun.com/geometry/construct-equitriangle.html

2. Draw or construct the altitude from the top vertex to form two congruent triangles.

3. Find the measure of the two angles at the top vertex and the length of the shorter leg.

*The top angles are each* *and the shorter leg is 1 in because the altitude of an equilateral triangle is also the angle and perpendicular bisector.*

4. Find the length of the longer leg, using the Pythagorean Theorem. Simplify the radical.

5. Now, let’s say the shorter leg is length

**30-60-90 Theorem:** If a triangle has angle measures

The shortest leg is always

**Example 3:** Find the length of the missing sides.

a)

b)

**Solution:** In part a, we are given the shortest leg and in part b, we are given the hypotenuse.

a) If

b) Now,

**Example 4:** Find the value of

a)

b)

**Solution:** In part a, we are given the longer leg and in part b, we are given the hypotenuse.

a) \begin{align*}x \sqrt{3} = 12\!\\ x = \frac{12}{\sqrt{3}} \cdot \frac{\sqrt{3}}{\sqrt{3}} = \frac{12 \sqrt{3}}{3} = 4 \sqrt{3}\!\\ \text{The hypotenuse is}\\ y = 2(4 \sqrt{3}) = 8 \sqrt{3}\end{align*}

b) \begin{align*}2x=16\!\\ x = 8\!\\ \text{The longer leg is}\!\\ y = 8 \cdot \sqrt 3 = 8 \sqrt{3}\end{align*}

**Example 5:** A rectangle has sides 4 and \begin{align*}4 \sqrt{3}\end{align*}. What is the length of the diagonal?

**Solution:** If you are not given a picture, draw one.

The two lengths are \begin{align*}x, x \sqrt{3}\end{align*}, so the diagonal would be \begin{align*}2x\end{align*}, or \begin{align*}2(4) = 8\end{align*}.

If you did not recognize this is a 30-60-90 triangle, you can use the Pythagorean Theorem too.

\begin{align*}4^2 + \left( 4 \sqrt{3} \right )^2 &= d^2\\ 16 + 48 &= d^2\\ d &= \sqrt{64} = 8\end{align*}

**Example 6:** A square has a diagonal with length 10, what are the sides?

**Solution:** Draw a picture.

We know half of a square is a 45-45-90 triangle, so \begin{align*}10=s \sqrt{2}\end{align*}.

\begin{align*}s \sqrt{2} &= 10\\ s &= \frac{10}{\sqrt{2}} \cdot \frac{\sqrt{2}}{\sqrt{2}}= \frac{10 \sqrt{2}}{2}=5 \sqrt{2}\end{align*}

**Know What? Revisited** The distance between \begin{align*}1^{st}\end{align*} and \begin{align*}3^{rd}\end{align*} base is one of the diagonals of the square. So, it would be the same as the hypotenuse of a 45-45-90 triangle. Using our ratios, the distance is \begin{align*}90 \sqrt{2} \approx 127.3 \ ft\end{align*}. The distance between \begin{align*}2^{nd}\end{align*} base and home plate is the same length.

## Review Questions

- Questions 1-4 are similar to Example 1-4.
- Questions 5-8 are similar to Examples 5 and 6.
- Questions 9-23 are similar to Examples 1-4.
- Questions 24 and 25 are a challenge.

- In an isosceles right triangle, if a leg is 4, then the hypotenuse is __________.
- In a 30-60-90 triangle, if the shorter leg is 5, then the longer leg is __________ and the hypotenuse is ___________.
- In an isosceles right triangle, if a leg is \begin{align*}x\end{align*}, then the hypotenuse is __________.
- In a 30-60-90 triangle, if the shorter leg is \begin{align*}x\end{align*}, then the longer leg is __________ and the hypotenuse is ___________.
- A square has sides of length 15. What is the length of the diagonal?
- A square’s diagonal is 22. What is the length of each side?
- A rectangle has sides of length 6 and \begin{align*}6 \sqrt{3}\end{align*}. What is the length of the diagonal?
- Two (opposite) sides of a rectangle are 10 and the diagonal is 20. What is the length of the other two sides?

For questions 9-23, find the lengths of the missing sides. Simplify all radicals.

** Challenge** For 24 and 25, find the value of \begin{align*}y\end{align*}. You may need to draw in additional lines. Round all answers to the nearest hundredth.

## Review Queue Answers

- \begin{align*}4^2+4^2 = x^2\!\\ {\;} \quad \ \ 32 = x^2\!\\ {\;} \qquad \ x = 4 \sqrt{2}\end{align*}
- \begin{align*}3^2+y^2 = 6^2\!\\ {\;} \qquad y^2 = 27\!\\ {\;} \qquad \ y = 3 \sqrt{3}\end{align*}
- \begin{align*}x^2 + x^2 = \left ( 10 \sqrt{2} \right )^2\!\\ {\;} \quad \ 2x^2 = 200\!\\ {\;} \qquad x^2 = 100\!\\ {\;} \qquad \ x = 10\end{align*}
- Yes, \begin{align*}9^2 + 12^2 = 15^2 \rightarrow 81+144 = 225\end{align*}
- Yes, \begin{align*}3^2 + \left( 3 \sqrt{3} \right )^2 = 6^2 \rightarrow 9+27 = 36\end{align*}

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