What if you knew that an angle was split exactly in half? How could you use this information to help you solve problems? After completing this Concept, you'll be able to bisect an angle and solve problems related to angle bisectors.

### Watch This

CK-12 Foundation: Chapter1CongruentAnglesandAngleBisectorsA

Watch the first part of this video.

James Sousa: Angle Bisector Exercise 2

### Guidance

When two rays have the same endpoint, an angle is created.

Here,
and
meet to form an angle. An angle is labeled with an “
” symbol in front of the three letters used to label it. This angle can be labeled
or
.
**
Always put the vertex (the common endpoint of the two rays) in the middle of the three points.
**
It doesn’t matter which side point is written first.

An
**
angle bisector
**
is a ray that divides an angle into two congruent angles, each having a measure exactly half of the original angle. Every angle has exactly one angle bisector.

is the angle bisector of

Label equal angles with
**
angle markings
**
, as shown below.

#####
**
Investigation:
**
Constructing an Angle Bisector

- Draw an angle on your paper. Make sure one side is horizontal.
- Place the pointer on the vertex. Draw an arc that intersects both sides.
- Move the pointer to the arc intersection with the horizontal side. Make a second arc mark on the interior of the angle. Repeat on the other side. Make sure they intersect.
- Connect the arc intersections from #3 with the vertex of the angle.

To see an animation of this construction, view http://www.mathsisfun.com/geometry/construct-anglebisect.html .

#### Example A

How many angles are in the picture below? Label each one two different ways.

There are three angles with vertex . It might be easier to see them all if we separate them out.

So, the three angles can be labeled, or or , and or .

#### Example B

What is the measure of each angle?

From the picture, we see that the angles are congruent, so the given measures are equal.

To find the measure of , plug in to .

Because too.

#### Example C

Is the angle bisector of ? If , what is and ?

Yes, is the angle bisector of according to the markings in the picture. If and , then . The and are each half of or .

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter1CongruentAnglesandAngleBisectorsB

### Vocabulary

When two geometric figures have the same shape and size then they are
**
congruent
**
. An

**is a ray that divides an angle into two congruent angles, each having a measure exactly half of the original angle.**

*angle bisector*### Guided Practice

For exercises 1 and 2, copy the figure below and label it with the following information:

1.

2.

3. Use algebra to determine the value of d:

**
Answers:
**

1. You should have corresponding markings on and .

2. You should have corresponding markings on and (that look different from the markings you made in #1).

3. The square marking means it is a angle, so the two angles are congruent. Set up an equation and solve:

### Interactive Practice

### Practice

For 1-4, use the following picture to answer the questions.

- What is the angle bisector of ?
- What is ?
- What is ?
- What is ?

For 5-6, use algebra to determine the value of variable in each problem.

For 7-10, decide if the statement is true or false.

- Every angle has exactly one angle bisector.
- Any marking on an angle means that the angle is .
- An angle bisector divides an angle into three congruent angles.
- Congruent angles have the same measure.

In Exercises 11-15, use the following information: is in the interior of . is in the interior of . is in the interior of . is in the interior of and and .

- Make a sketch.
- Find
- Find
- Find
- Find