**Learning Goal**

By the end of the lesson I will be able to . . . describe the properties and relationships of the exterior angles of triangles

What if you knew that two of the exterior angles of a triangle measured ? How could you find the measure of the third exterior angle? After completing this Concept, you'll be able to apply the Exterior Angle Sum Theorem to solve problems like this one.

### Watch This

CK-12 Exterior Angles Theorems

James Sousa: Introduction to the Exterior Angles of a Triangle

Then watch this video.

James Sousa: Proof that the Sum of the Exterior Angles of a Triangle is 360 Degrees

Finally, watch this video.

James Sousa: Proof of the Exterior Angles Theorem

### Guidance

An **Exterior Angle** is the angle formed by one side of a polygon and the extension of the adjacent side.

In all polygons, there are **two** sets of exterior angles, one that goes around clockwise and the other goes around counter-clockwise.

Notice that the interior angle and its adjacent exterior angle form a linear pair and add up to .

There are two important theorems to know involving exterior angles: the Exterior Angle Sum Theorem and the Exterior Angle Theorem.

The **Exterior Angle Sum Theorem** states that the exterior angles of any polygon will always add up to .

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The **Exterior Angle Theorem** states that an exterior angle of a triangle is equal to the sum of its remote interior angles. (**Remote Interior Angles** are the two interior angles in a triangle that are not adjacent to the indicated exterior angle.)

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#### Example A

Find the measure of .

Notice that is an exterior angle of and is supplementary to .

Set up an equation to solve for the missing angle.

#### Example B

Find the measures of the numbered interior and exterior angles in the triangle.

We know that because they form a linear pair. So, .

Similarly, because they form a linear pair. So, .

We also know that the three interior angles must add up to by the Triangle Sum Theorem.

#### Example C

What is the value of in the triangle below?

First, we need to find the missing exterior angle, which we will call . Set up an equation using the Exterior Angle Sum Theorem.

and add up to because they are a linear pair.

CK-12 Exterior Angles Theorems

### Guided Practice

1. Find .

2. Two interior angles of a triangle are and . What are the measures of the three exterior angles of the triangle?

3. Find the value of and the measure of each angle.

**Answers:**

1. Using the Exterior Angle Theorem

If you forget the Exterior Angle Theorem, you can do this problem just like Example C.

2. Remember that every interior angle forms a linear pair (adds up to ) with an exterior angle. So, since one of the interior angles is that means that one of the exterior angles is (because ). Similarly, since another one of the interior angles is , one of the exterior angles must be . The third interior angle is not given to us, but we could figure it out using the Triangle Sum Theorem. We can also use the Exterior Angle Sum Theorem. If two of the exterior angles are and , then the third Exterior Angle must be since .

So, the measures of the three exterior angles are , and .

3. Set up an equation using the Exterior Angle Theorem.

Substitute in for to find each angle.

### Practice

Determine .

Use the following picture for the next three problems:

- What is ?
- What is ?
- What is ?

Solve for .