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Angle Measurement

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Angle Measurement

What if you were given the measure of an angle and two unknown quantities that make up that angle? How would you find the values of those quantities? After completing this Concept, you'll be able to use the Angle Addition Postulate to evaluate such quantities.

Watch This

CK-12 Measuring Angles

James Sousa: Animation of Measuring Angles with a Protractor

Then look at the first part of this video.

James Sousa: Angle Basics

Guidance

An angle is formed when two rays have the same endpoint. The vertex is the common endpoint of the two rays that form an angle. The sides are the two rays that form an angle.

Label It Say It
\angle ABC Angle ABC
\angle CBA Angle CBA

The vertex is B and the sides are \overrightarrow{BA} and \overrightarrow{BC} . Always use three letters to name an angle, \angle SIDE-VERTEX-SIDE.

Angles are measured with something called a protractor . A protractor is a measuring device that measures how “open” an angle is. Angles are measured in degrees and are labeled with a ^\circ symbol. For now, angles are always positive.

There are two sets of measurements, one starting on the left and the other on the right side of the protractor. Both go around from 0^\circ to 180^\circ . When measuring angles, you can line up one side with 0^\circ , and see where the other side hits the protractor. The vertex lines up in the middle of the bottom line.

Note that if you don't line up one side with 0^\circ , the angle's measure will be the difference of the degrees where the sides of the angle intersect the protractor.

Sometimes you will want to draw an angle that is a specific number of degrees. Follow the steps below to draw a 50^\circ angle with a protractor:

1. Start by drawing a horizontal line across the page, 2 in long.

2. Place an endpoint at the left side of your line.

3. Place the protractor on this point, such that the line passes through the 0^\circ mark on the protractor and the endpoint is at the center. Mark 50^\circ on the appropriate scale.

4. Remove the protractor and connect the vertex and the 50^\circ mark.

This process can be used to draw any angle between 0^\circ and 180^\circ . See http://www.mathsisfun.com/geometry/protractor-using.html for an animation of this.

When two smaller angles form to make a larger angle, the sum of the measures of the smaller angles will equal the measure of the larger angle. This is called the Angle Addition Postulate . So, if B is on the interior of \angle ADC , then m \angle ADC = m \angle ADB + m \angle BDC .

Example A

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

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

So, the three angles can be labeled, \angle XUY (or \angle YUX ), \angle YUZ (or \angle ZUY ), and \angle XUZ (or \angle ZUX ).

Example B

Measure the three angles from Example 1, using a protractor.

Just like in Example A, it might be easier to measure these three angles if we separate them.

With measurement, we put an m in front of the \angle sign to indicate measure. So, m\angle XUY = 84^\circ, \ m\angle YUZ = 42^\circ and m\angle XUZ = 126^\circ .

Example C

What is the measure of the angle shown below?

This angle is lined up with 0^\circ , so where the second side intersects the protractor is the angle measure, which is 50^\circ .

CK-12 Measuring Angles

Guided Practice

1. What is the measure of the angle shown below?

2. Use a protractor to measure \angle RST below.

3. What is m \angle QRT in the diagram below?

Answers

1. This angle is not lined up with 0^\circ , so use subtraction to find its measure. It does not matter which scale you use.

Inner scale: 140^\circ - 15^\circ = 125^\circ

Outer scale: 165^\circ - 40^\circ = 125^\circ

2. Lining up one side with 0^\circ on the protractor, the other side hits 100^\circ .

3. Using the Angle Addition Postulate, m \angle QRT = 15^\circ + 30^\circ = 45^\circ .

Practice

1. What is m \angle LMN if m \angle LMO = 85^\circ and m \angle NMO = 53^\circ ?

2. If m\angle ABD = 100^\circ , find x .

For questions 3-6, determine if the statement is true or false.

  1. For an angle \angle ABC, C is the vertex.
  2. For an angle \angle ABC, \overline{AB} and \overline{BC} are the sides.
  3. The m in front of m \angle ABC means measure.
  4. The Angle Addition Postulate says that an angle is equal to the sum of the smaller angles around it.

For 7-12, draw the angle with the given degree, using a protractor and a ruler.

  1. 55^\circ
  2. 92^\circ
  3. 178^\circ
  4. 5^\circ
  5. 120^\circ
  6. 73^\circ

For 13-16, use a protractor to determine the measure of each angle.

Solve for x .

  1. m\angle ADC = 56^\circ

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