One way to measure the size of an angle is in degrees. To indicate the measure of an angle, write a lowercase m in front of the angle symbol. In the diagram, the measure of ∠ A angle eh  is 62. You write this as m ∠ A = 62 . m angle . eh equals 62 .  In this book, you will work only with degree measures.

A circle has 360 ° , 360 degrees comma  so 1 degree is 1 360 1 over 360  of a circle. A protractor forms half a circle and measures angles from 0 ° 0 degrees  to 180 ° . 180 degrees .

The Protractor Postulate allows you to find the measure of an angle. Consider the diagram below. The measure of ∠ COD angle  is the absolute value of the difference of the real numbers paired with O C → o c vector  and O D → . o d vector , .  That is, if O C → o c vector  corresponds with c, and O D → o d vector  corresponds with d, then m ∠ COD = | c − d | . m angle

Notice that the Protractor Postulate and the calculation of an angle measure are very similar to the Ruler Postulate and the calculation of a segment length.

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