# Bisectors of Triangles

As you well know by now, being able to deduce key information from a limited set

of facts is the basis of geometry.

An important type of segment, ray, or line that can help us prove congruence is

called an **angle bisector**. Understanding what angle bisectors are and how

they affect triangle relationships is crucial as we continue our study of geometry.

Let’s investigate different types of bisectors and the theorems that accompany them.

## Perpendicular Bisector

A **perpendicular bisector** is a special kind of segment, ray, or line that

**(1) intersects a given segment at a 90° angle, and**

**(2) passes through the given segment’s midpoint.**

*Segment CD is the perpendicular bisector to segment AB.*

We derive two important theorems from the characteristics of perpendicular bisectors.

We can use these theorems in our two-column geometric proofs, or we can just use

them to help us in geometric computations.

### Perpendicular Bisector Theorem

If a point lies on the perpendicular bisector of a segment, then it is equidistant

from the endpoints of the segment.

*Converse also true: If a point is equidistant from the endpoints of a segment, then
it lies on the perpendicular bisector of the segment.*

These theorems essentially just show that there exist a locus of points (which form

the perpendicular bisector) that are equidistant from the endpoints of a given segment

which meet at the midpoint of the segment at a right angle. An illustration of this

concept is shown below.

*Points E, F, G, and H (along with an infinite amount of points) are equidistant from
A and B. Together, they form the perpendicular bisector of segment AB.*

The perpendicular bisectors of a triangle have a very special property. Let’s investigate

it right now.

### Circumcenter Theorem

The perpendicular bisectors of the sides of a triangle intersect at a point called

the **circumcenter** of the triangle, which is equidistant from the vertices

of the triangle.

*Point G is the circumcenter of ?ABC.*

## Angle Bisectors

Now, we will study a geometric concept that will help us prove congruence between

two angles. Any segment, ray, or line that divides an angle into two congruent angles

is called an **angle bisector**.

We will use the following angle bisector theorems to derive important information

from relatively simple geometric figures.

### Angle Bisector Theorem

If a point lies on the bisector of an angle, then it is equidistant from the sides

of the angle.

*Converse also true: If a point in the interior of an angle is equidistant from the
sides of the angle, then it lies on the bisector of the angle.*

*The points along ray AD are equidistant from either side of the angle. Together,
they form a line that is the angle bisector.*

Similar to the perpendicular bisectors of a triangle, there is a common point at

which the angle bisectors of a triangle meet. Let’s look at the corresponding theorem.

### Incenter Theorem

The angle bisectors of a triangle intersect at a point called the **incenter**

of the triangle, which is equidistant from the sides of the triangle.

*Point G is the incenter of ?ABC.*

## Summary

While similar in many respects, it will be important to distinguish between perpendicular

bisectors and angle bisectors. We use perpendicular bisectors to create a right

angle at the midpoint of a segment. Any point on the perpendicular bisector is equidistant

from the endpoints of the given segment. The point at which the perpendicular bisectors

of a triangle meet, or the circumcenter, is equidistant from the vertices of the

triangle.

On the other hand, angle bisectors simply split one angle into two congruent angles.

Points on angle bisectors are equidistant from the sides of the given angle. We

also note that the points at which angle bisectors meet, or the incenter of a triangle,

is equidistant from the sides of the triangle.

Let’s work on some exercises that will allow us to put what we’ve learned about

perpendicular bisectors and angle bisectors to practice.

### Exercise 1

** BC** is the perpendicular bisector of

**. Find the value**

*AD*of

**.**

*x*

**Solution:**

The most important fact to notice is that ** BC** is the perpendicular

bisector of

**because, although it is just one statement, we can**

*AD*derive much information about the figure from it. The fact that it is a perpendicular

bisector implies that segment

**is equal to segment**

*DB*

*AB*since it passes through the midpoint of segment

**. Therefore, we**

*AD*have

Subtracting ** 7x** from both sides of the equation yields

So, we have ** x=6**.

### Exercise 2

** N** is the circumcenter of

**. Find the values of**

*?RAK***and**

xx

**.**

*y*

**Solution:**

We are aware that segments ** RM**,

**, and**

*XS*

*YE*are all perpendicular bisectors since they meet at

**, the circumcenter**

*N*of

**. Thus, in order to solve for**

*?RAK***, we can set segments**

*x***and**

*RE***equal to each other since**

*AE*

*E*is the midpoint of segment

**. We have**

*RA*

In order to solve for ** y**, we have to use the information given by the

**Circumcenter Theorem**. This theorem states that the circumcenter is equidistant

from the vertices of the triangle. Thus, we know that

**. For**

*RN=AN=KN*this part of the problem, we only need to solve for

**with**

*y***.**

*AN=KN*We have

So, we have ** x=4** and

**.**

*y=3*### Exercise 3

Find the value of * x*.

**Solution:**

The illustration shows that points * A* and

*are equidistant*

**B**from point

*. By the converse of the*

**L****Angle Bisector Theorem**,

we know that

*must lie on the angle bisector of*

**L***.*

**?AYB**This means that

*, so we can solve for*

**?AYL=?BYL***as*

**x**shown below:

So, our answer is ** x=4**.

### Exercise 4

** QS** is the angle bisector of

**. Find the value of**

*?PQR***.**

*x*

From the information we’ve been given, we know that ** ?PQS** is congruent

to

**because**

*?SQR***bisects the whole angle,**

*QS***.**

*?PQR*We have been given the measure of the whole angle and the measure of

**,**

*?SQR*which is half of the entire angle (since the angle has been bisected). Therefore,

we have

Thus, we get ** x=9**. If we were to plug in

**for**

*9***,**

*x*we would see that, indeed,

**is half of**

*?SQR***.**

*?PQR*