- 1). Count the number of sides of the polygon.
- 2). Subtract 2 from the number of sides of the polygon. For example, if you have a pentagon, or five-sided figure, you would subtract 2 from 5 to get 3.
- 3). Multiply the result from Step 2 by 180 to calculate the number of degrees of all the angles in the polygon. Continuing with the example, you would multiply 3 by 180 to get 540 total degrees.
- 4). Add the total of the known angles. In the example, if you knew four of the angles in the pentagon were 93, 100, 105 and 120, you would add these four angles to get 418.
- 5). Subtract the measures of the known angles from the total degrees of the polygon to find the missing angle. Finishing the example, you would subtract 418 from 540 to find the missing angle would be 122 degrees.
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