# Angle Properties of Triangles

Now that we are acquainted with the classifications

of triangles, we can begin our extensive study of the angles of triangles.

In many cases, we will have to utilize the angle theorems

we’ve seen to help us solve problems and proofs. However, there are some triangle

theorems that will be just as essential to know. This first theorem tells

us that if we know the measures of two angles of a triangle, it is possible to determine

the measure of the third angle.

## Triangle Angle Sum Theorem

The sum of the measures of the interior angles of a triangle is 180.

*The diagram above illustrates the Triangle Angle Sum Theorem.*

Let’s do some examples involving the Triangle Sum Theorem to help us see its utility.

### Examples

**(1) Find the measure of ?C.**

**Solution:**

As with all problems, we must first use the facts that are given to us. Using the

diagram, we are given that

Since our goal is to find the measure of ** ?C**, we can use the Triangle

Angle Sum Theorem to solve for the missing angle. So we have

Using the angle measures we were given, we can substitute those values into our

equation to get.

Having ** ?C** measure out to

**satisfies the property**

*26°*that the sum of the interior angles of a triangle is

**.**

*180°*
**(2) Find the value of x in the diagram below.**

**Solution:**

In this exercise, we are given that

Looking at ** ?RST**, we see that two of three angles are given to us.

Thus, we can apply the Triangle Angle Sum Theorem to figure out the measure of the

third angle:

Note that * ?SRT* is the vertical angle opposite

**,**

*?QRP*so we can deduce that

Then, by the definition of congruent angles, we have

Now, we have one of three angle measures of ** ?QRP**. Since we know that

*, we can use the Triangle Angle Sum Theorem as follows*

**m?P = m?Q = x**

We have found the measure of ** ?P** and

**to be**

*?Q***.**

*67*
In order to comprehend the next theorem, we must learn two more terms that describe

angles. The angle formed by one side of a triangle with the extension of another

side is called an **exterior angle** of the triangle.

*Exterior angles get their name because they lie on the outsides of triangles.*

The two angles that are not adjacent, or next to, the exterior angle of the triangle

are called **remote interior angles**.

Now that we know what these terms mean, we are ready for a theorem that will help

us tremendously in our proofs.

## Exterior Angle Theorem

The measure of an exterior angle of a triangle is equal to the sum of the measures

of the two remote interior angles.

*Adding the measures of the two remote interior angles of a triangle gives the measure
of the exterior angle.*

Let’s see how the Exterior Angle Theorem can be utilized to help us find the measures

of unknown angles in the examples below.

### Examples

**(1) Find the measures of ?1 and ?2 in the figure below.**

**Solution:**

First, we can solve for ** m?1** since we are given the measure of two

angles of that triangle. This part of the problem is similar to the examples we

have already done above. Let’s begin with the statements of what we are given, which

are:

Now, we can solve for ** m?1** by using the Triangle Angle Sum Theorem.

So we have

In order to solve for the measure of ** ?2**, we will need to apply the

Exterior Angle Theorem. We know that the two remote interior angles in the figure

are

**and**

*?S***. Thus, by the Exterior Angle Theorem,**

*?A*the sum of those angles is equal to the measure of the exterior angle. We have

While not always necessary, we can check our solution using our previous knowledge

of lines. We see that ** ?1** and

**make up ray**

*?2***.**

*AK*And since straight lines have

*measures, we know that the sum*

**180°**of

**and**

*?1***must be**

*?2**. Let’s check*

**180**to make sure:

So, we know we have worked this problem out correctly.

**(2) Find m?B.**

**Solution:**

Let’s take a look at the information we have been given first. We know that

Right off the bat, we can apply the Exterior Angle Theorem to help us solve the

problem. We have

This does not answer the question, however. The question asked for ** m?B**.

The variable

**alone does not tell us what the measure of the angle**

*x*is. So, we must plug

**into our equation for**

*x = 4***:**

*m?B*

.

Now we have found that the measure of ** ?B** is

**.**

*39°*