# Isosceles and Equilateral Triangles

In the first section of our study of triangles, we learned how to classify triangles

by the measures of their angles and by the lengths of their sides. However, aside

from the names we give triangles, it will be important to understand and recognize

the characteristics that make certain triangles special. In this section, we will

take a closer look at the properties of some unique figures: **isosceles** and

**equilateral triangles**.

## Isosceles Triangles

Let’s begin our study of isosceles triangles by learning new terminology that will

help us identify various characteristics of these kinds of triangles. Recall that

an isosceles triangle is a triangle with at least two congruent sides. These congruent

sides are called **legs**. The point at which these legs meet is called the **vertex
point** of the isosceles triangle, and the angle formed by the legs is called

the

**vertex angle**. The other two angles of the triangle are called

**base angles**.

A labeled illustration of an isosceles triangle is shown below.

In the past, we determined isosceles triangles by the lengths of their sides. In

other words, if we saw that a triangle had two sides with equal lengths, then we

classified the triangle as isosceles. There are other characteristics that mark

isosceles triangles, however. Let’s look at an important theorem that gives us even

more information about these kinds of triangles.

### Isosceles Triangle Theorem

*If two sides of a triangle are congruent, then the angles opposite those sides are
congruent.*

*Converse also true: If two angles of a triangle are congruent, then the sides opposite
those angles are congruent.*

We will practice using these theorems to help us solve the following exercises.

## Exercise 1

**Determine the values of x and y.**

**Solution:**

In the diagram, we are given that ** ?A** is

**. Because**

*52°*the side opposite of

**is congruent to the side opposite of**

*?A***,**

*?C*we know that the angles are also congruent by the

**Isosceles Triangle Theorem**.

Thus, the value of

**is**

*y***.**

*52*
Now, let’s try to determine the value of ** x**. In order to figure this

out, we must use the

**Triangle Angle Sum Theorem**to figure out what the total

degree measure is at

**.**

*?B*

Since we’ve determined that ** ?B** must have a measure of

**,**

*76°*we can write an algebraic equation to help us solve for

**. This method**

*x*is shown below.

We subtract ** 6** from both sides of the equation.

Now, we divide by ** 14** to find

**.**

*x*

Our answers are ** x = 14** and

**.**

*y = 52*## Exercise 2

**Solution:**

Let’s look at the information we’ve been given to see which direction we’d like

to take this problem in. We are given that ** ?TUS** and

*?QSR*are congruent.

Now, let’s try to find a special relationship that either ** ?TUS** or

**may have with another angle in the diagram. Notice that**

*?QSR*

*?QSR*and

**are vertical angles, so by the**

*?TSU***Vertical Angles Theorem**,

we can say that they are congruent to each other.

We can now apply the **Transitive Property** to show that *?TUS*

and ** ?TSU** are congruent.

Finally, by the **Isosceles Triangle Theorem**, we know that the sides opposite

of two congruent angles are also congruent. Thus, segments ** TS** and

**are congruent to each other. Our new diagram and the two-column**

*TU*geometric proof for this exercise are shown below.

## Exercise 3

**Determine the values of x and y in the figure below.**

**Solution:**

We first want to notice that ** ?BCA** and

**are supplementary.**

*?BCD*Recall, that this means that their sum of their degree measures is

**.**

*180*Thus we will try to determine the measure of

**:**

*?BCA*

By the **Triangle Angle Sum Theorem**, we know that the sum of ** ?A**,

**, and**

*?B***is**

*?BCA***, so we will try to**

*180°*determine the values of

**and**

*x***by figuring out what**

*y*the sum of

**and**

*?A***should be.**

*?B*

Together, ** ?A** and

**should have a measure of**

*?B***.**

*124*
Let’s look at the diagram again. Notice that segment ** AC** is congruent

to segment

**. So, by the**

*BC***Isosceles Triangle Theorem**, we know

that

**is congruent to**

*?A***(since they are the angles**

*?B*opposite of the congruent sides). Therefore, we can divide the remainder of the

angle measures of the triangle,

**, by the two congruent angles to**

*124*determine what the measure of each angle should be. When we do this, we see that

**and**

*?A***should come out to**

*?B***each.**

*62°*
To solve for ** x**, we have

To solve for ** y**, we have

So, we have ** x = 31** and

**.**

*y = 4*## Equilateral Triangles

Equilateral triangles are another type of triangle with unique characteristics.

Knowledge of these kinds of triangles will assist us in some of the proofs and exercises

we will encounter in the future, so let’s take a closer look at the traits that

make equilateral triangles special.

While the following characteristics of equilateral triangles are not theorems or

postulates, they are statements we can use in our proofs. The following statements

are called **corollaries**. Corollaries are proven results that rely heavily

on one theorem. The following corollaries of equilateral triangles are a result

of the **Isosceles Triangle Theorem**:

**(1) A triangle is equilateral if and only if it is equiangular.**

**(2) Each angle of an equilateral triangle has a degree measure of 60.**

*The congruent sides of the triangle imply that all the angles are congruent. We can
also use the converse of this, which is that three congruent angles imply three
congruent sides in a triangle. Each of the angles above is 60°.*

Let’s practice using these corollaries in the following exercises.

## Exercise 4

**Determine the values of x and y in the diagram below.**

**Solution:**

In order to solve this problem, we must recognize the fact that the triangle shown

is an equilateral triangle. We notice this by the tick marks on all three sides

of the triangle. This indicates to us that all three sides of the triangle are congruent.

Moreover, we must be able to understand the relationship between the angles of the

triangle. In order to solve for ** x**, we will need to keep in consideration

that every angle of an equilateral triangle is

**.**

*60°*
We will solve for ** x** first. In order to do this, we need to use the

information given to us about the sides of the triangles to solve for

**.**

*x*We will set

**equal to**

*2(2x + 1)***since equilateral**

*14*triangles have congruent sides. Thus, we have

Now that we have solved for ** x**, let’s determine the value for

**.**

*y*This part of the exercise requires our knowledge of the angles of equilateral triangles.

As mentioned before, every angle has a measure of

**, so we have**

*60*

We have already determined the value of ** x**, so we can plug this value

right into our equation to solve for

**.**

*y*

Thus, we get ** x = 3** and

**.**

*y = 6*## Exercise 5

**Solution:**

First, we will consider the information we’ve been given to see if we can derive

any more useful information from it. We are given that ** ?RQS** and

**are congruent, as shown in the diagram. Also, we are told that**

?TQS?TQS

**is an equilateral triangle. This fact will be of use to us as**

*?RQT*we continue the exercise.

Since ** ?RQT** is an equilateral triangle, we know that all three sides

and angles of the triangle are congruent. Thus, we can say that segments

*RQ*and

**are congruent to each other.**

*TQ*
Now, we have one pair of sides and one pair of angles that are congruent to each

other. If we can prove that one more pair of corresponding sides of *?RQS*

and ** ?TQS** are congruent, then we can use the

**SAS Postulate**to

prove that the triangles are congruent. Indeed, if we use the

**Reflexive Property**

to show that

**is congruent to itself, we see that the two triangles**

*QS*are congruent to each other. Now, our figure looks like this:

Finally, we can say that segment ** RS** is congruent to segment

*TS*because they are corresponding sides of congruent triangles, so they are congruent.

Our two-column proof is shown below.