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Angle Measurement

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Angle Measurement
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Lesson 1.5: Angle Measurement

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.

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.


Ticket In/Ticket Out Activity:

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.

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.

Warm-Up Activity/Guided Practice:

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

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

3. If m \angle ABD = 100^\circ , find x and m \angle ABC and m \angle CBD ?

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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