How can the contractor know what the measurement of the third angle should be before he measures it? A contractor works on a house. He buys small boards of wood to trim the outline of the triangular shaped small roof over the door. After the pieces are in place, the contractor measures the angles to make sure that the trim is a perfect triangle. The first angle measures

In this concept, you will learn about the sum of the angles of triangles.

### Triangle Sum Theorem

A **triangle** is a figure with three sides and three angles.

In a triangle there is a relationship between the **interior angles** of the triangle. Interior angles are the angles inside the triangle.

Notice that triangles

Now let’s look at triangles a little differently. In geometry, a triangle can be formed by the intersection of three lines.

First, notice that the lines create the three interior angles of the triangle.

Next notice that if you extend any side of the triangle, then it stretches beyond the triangle. This creates a pair of angles, an interior angle and an **exterior angle .** An exterior angle is the angle formed outside of the edge of the triangle.

Here is a clearer example of an exterior angle.

As you can see, the interior angle and the exterior angle form a line. Therefore their sum must be

The adjacent angle to the interior angle is

The missing measure of the interior angle is

There is another way to calculate the value of the missing interior angle. Take a look at the other given interior angles of the triangle. They are

### Examples

#### Example 1

Earlier, you were given a problem about the contractor and the triangle shaped trim.

If the first angle is

First, determine if you are calculating an interior or exterior angle.

The third angle is the interior angle.

Next, remember the relationship between the three interior angles.

The third angle +

Then determine the measure of the third angle.

#### Example 2

Solve the following problem.

What is the measure of angle

First, determine if angle

Angle

Next, remember the relationship between the exterior angle of a missing angle and the two provided interior angles.

The other two interior angles have a sum equal to the measure of the third angle's exterior angle.

Then, calculate the value of angle

Angle

The answer is that the measure of angle

#### Example 3

If the sum of two angles of a triangle is ** 150∘,** then what is the measure of the third angle?

First, determine if you are calculating an interior or exterior angle.

Interior

Next, remember the relationship between interior angles.

The sum of three interior angles is

Then, calculate the sum of the third angle.

The answer is that the third angle is

#### Example 4

If the sum of two of the angles is ** 75∘,** then what is the measure of the third angle’s exterior angle?

First, determine if you are calculating an interior or exterior angle.

Exterior

Next, remember the relationship between two interior angles and the measure of the third angle's exterior angle.

The sum of the two interior angles is equal to the measure of the third angle's exterior angle.

Then, determine the measure of the third angle.

The answer is that the measure of the third angle's exterior angle is

#### Example 5

A triangle has three angles: angle

First, determine if you are calculating an interior or exterior angle.

Angle

Next, remember the relationship between the three interior angles.

Angle

Then, determine the measure of the third angle.

The answer is that the measure of angle

### Review

Find the measure of the missing angle in each triangle.

20+70+x=180∘ 60+60+x=180∘ 90+15+x=180∘ 100+45+x=180∘ 10+105+x=180∘ 120+45+x=180∘ 145+5+x=180∘ 150+20+x=180∘ 110+45+x=180∘ 60+40+x=180∘ 70+50+x=180∘ 80+45+x=180∘ 50+45+x=180∘ 30+55+x=180∘ 75+55+x=180∘

### Review (Answers)

To see the Review answers, open this PDF file and look for section 8.7.