In a geometry class--and in any activity requiring geometry skills--being able to measure angles precisely and accurately is an essential skill. A tool called a protractor is used to measure an angle. The steps below demonstrate exactly how to use this unique tool.
Step 1
Begin by examining the angle and identifying its initial side. When you identify the initial side, you are naming the ray that you will use to line up your protractor. In this image, which ray is the angle's initial side? \(\mathsf{ \overrightarrow{AC} }\); since the diagram indicates \(\mathsf{ \overrightarrow{AB} }\) rotated counterclockwise from \(\mathsf{ \overrightarrow{AC} }\) Step 2
Next, place your protractor along the initial side of the angle so that vertex A is at the center of the straight-edge of the protractor. The initial side, ray AC, points to the 0 on the protractor. Note that the curved edge is labeled with the numbers 0 to 180, going clockwise, and 180 to 0 going counterclockwise. You need the numbers to go in both directions in case your angle is oriented differently than this angle. You may also notice that your protractor has a ruler along its base. This ruler will not be used to measure angles.
Step 3
Check to make sure that your protractor's straight edge aligns perfectly with the angle's initial side. This will ensure that you will capture the correct measurement. Step 4
The final step is choosing which number on the curved side of the protractor to use. Which one should we use to measure this angle. How do you know? This angle measures 50°. We use 50 instead of 130 because the angle's base lines up with the 0 on the protractor, and all angles are measured using the row that is associated with 0° at the base of the angle. Following the numbers up the curve from 0 to the number 50, we can count consecutively by 10's: 0-10-20-30-40-50. The other leg, \(\small\mathsf{ \overrightarrow{AB} }\), crosses the protractor at 50°.
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