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Glencoe Geometry: Online Textbook Help13 chapters | 152 lessons

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Lesson Transcript

Instructor:
*Miriam Snare*

Miriam has taught middle- and high-school math for over 10 years and has a master's degree in Curriculum and Instruction.

In this lesson, you will learn the definition of the Angle Addition Postulate. We will look at some examples so that you understand how this postulate works.

The main idea behind the **Angle Addition Postulate** is that if you place two angles side by side, then the measure of the resulting angle will be equal to the sum of the two original angle measures. For this postulate to apply, the **vertices**, which are the corner points of the angle, have to also be placed together. We can illustrate this idea by using the heads of two arrows. We are going to label the arrowheads with some points to make it easier to name the angles.

In the diagram above, the tip of the blue arrow forms angle *BLU* which measures 30 degrees. The tip of the red arrow forms angle *RED* which measures 50 degrees. Now, we are going to add these two angles together by rotating the arrows so that one side of angle *BLU* is against one side of angle *RED* and the points at the tips (points *L* and *E*) come together. For simplicity, we will call the point at the tips just *L.*

Looking at the outside edges of the two arrowheads, there is a new angle that has been created: angle *RLU.* This angle has a measure of 80 degrees because it was created by joining a 30-degree angle and a 50-degree angle without any space between them and without overlapping them. Below, you can see angle *RLU* with just the shadows of the two added angles in it.

Earlier, we looked at how the Angle Addition Postulate physically combines two angles. In a geometry textbook, you often find the Angle Addition Postulate written like this:

If point *B* lies in the interior of angle *AOC,* then

Let's start by looking at the diagram to explain the first part of the theorem. Angle *AOC* is created by the two red rays. In the interior of angle AOC is point *B.* A ray is drawn from point *O* through *B,* which splits angle *AOC* into two parts (angle *AOB* and angle *BOC*). The formula in the theorem tells us that if we add the measures of the two parts (angle *AOB* and angle *BOC*) together, we get the measure of the big red angle (angle *AOC*).

Now, let's look at a couple of examples that apply the Angle Addition Postulate.

Example 1: Use the diagram below to find the measure of angle *GEM* if angle *GEO* measures 158 degrees and angle *MEO* is a right angle.

So, let's think through the Angle Addition Postulate. What is the name of the big angle in the diagram? What are the names of the two smaller angles that combine to create the big angle?

Angle *GEO* is the big angle that is made up of angles *GEM* and *MEO.* So, we can write the formula from the Angle Addition Postulate for these angles:

Angle *MEO* measures 90 degrees because it is a *right angle.* Angle *GEO* measures 158 degrees as was given in the problem statement. We can substitute in those values into the equation:

Now, we have an equation that we just need to solve for the measure of angle *GEM.* So, we subtract 90 from both sides. Therefore, angle *GEM* measures 68 degrees.

Let's try another example.

Example 2: Use the diagram below to find the value of *x.*

This problem seems to give us a lot less information than the first example because we are not given what any of the angles actually measure. Instead, we are given algebraic expressions for two angles. However, the diagram gives us additional information that the problem statement did not. Let's break this problem down the same way we did the first one. What is the name of the big angle? What are the names of the two angles that combine to create the big angle?

The two smaller angles are angle *ABD* and angle *DBC.* These two angles together form big angle *ABC.* In the diagram, we can see that angle *ABC* is a **straight angle**, which means it measures 180 degrees. For angle *ABD* and angle *DBC,* we are given algebraic expressions for the measures. We can write the formula from the Angle Addition Postulate for these angles and substitute in what we know about each angle's measure:

Now, we solve the equation. First, we combine the like terms on the left side:

Then we subtract 5 from both sides and divide by 7 to isolate *x*:

- The
**Angle Addition Postulate**states that: If point*B*lies in the interior of angle AOC, then - The postulate describes that putting two angles side by side with their
**vertices**together creates a new angle whose measure equals the sum of the measures of the two original angles. - A diagram is often helpful for setting up the formula from the Angle Addition Postulate for a particular problem.

- The Angle Addition Postulate states that the measure of an angle formed by two angles side by side is the sum of the measures of the two angles.
- The Angle Addition Postulate can be used to calculate an angle formed by two or more angles or to calculate the measurement of a missing angle.

Upon reaching the end of the lesson, display your ability to:

- State the Angle Addition Postulate
- Write the textbook definition of the postulate
- Use the Angle Addition Postulate to calculate the measure of an angle

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Glencoe Geometry: Online Textbook Help13 chapters | 152 lessons

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- Inductive & Deductive Reasoning in Geometry: Definition & Uses 4:59
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- Reflexive Property of Equality: Definition & Examples 3:43
- Symmetric Property of Equality: Definition & Examples 3:26
- Transitive Property of Equality: Definition & Example 3:39
- Angle Addition Postulate: Definition & Examples 5:15
- Go to Glencoe Geometry Chapter 2: Reasoning and Proof

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