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Angle Measurement

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Angle Measurement student textbook

What if you needed a way to describe the size of an angle? After completing this Concept, you'll be able to use a protractor to measure an angle in degrees.

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CK-12 Foundation: Chapter1MeasuringAnglesA

James Sousa: Animation of Measuring Angles with a Protractor

Watch the first part of this video.

James Sousa: Angle Basics

Guidance

We measure a line segment’s length with a ruler. 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 labeled with a ^\circ symbol.

Notice that there are two sets of measurements, one opening clockwise and one opening counter-clockwise, from 0^\circ to 180^\circ . When measuring angles, always 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, where all the degree lines meet.

For every angle there is a number between 0^\circ and 180^\circ that is the measure of the angle in degrees. The angle's measure is then the absolute value of the difference of the numbers shown on the protractor where the sides of the angle intersect the protractor. In other words, you do not have to start measuring an angle at 0^\circ , as long as you subtract one measurement from the other.

The Angle Addition Postulate states that if B is on the interior of \angle ADC , then m \angle ADC = m \angle ADB + m \angle BDC . See the picture below.

Drawing a 50^\circ Angle with a Protractor
  1. Start by drawing a horizontal line across the page, about 2 in long.
  2. Place an endpoint at the left side of your line.
  3. Place the protractor on this point. Make sure to put the center point on the bottom line of the protractor on the vertex. 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 investigation.

Copying an Angle with a Compass and Straightedge
  1. We are going to copy the angle created in the previous investigation, a 50^\circ angle. First, draw a straight line, about 2 inches long, and place an endpoint at one end.
  2. With the point (non-pencil side) of the compass on the vertex, draw an arc that passes through both sides of the angle. Repeat this arc with the line we drew in #1.
  3. Move the point of the compass to the horizontal side of the angle we are copying. Place the point where the arc intersects this side. Open (or close) the “mouth” of the compass so you can draw an arc that intersects the other side of the arc drawn in #2. Repeat this on the line we drew in #1.
  4. Draw a line from the new vertex to the arc intersections.

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

Example A

Measure the three angles using a protractor.

Example B

What is the measure of the angle shown below?

Example C

What is m \angle QRT in the diagram below?

Example D

Draw a 135^\circ angle.

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter1MeasuringAnglesB

Vocabulary

A protractor is a measuring device that measures how “open” an angle is. Angles are measured in degrees , and labeled with a ^\circ symbol. A compass is a tool used to draw circles and arcs.

Interactive Practice

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
  2. m \angle ADC = 130^\circ
  3. m \angle ADC = (16x - 55)^\circ
  4. m \angle ADC = ( 9x - 80)^\circ

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