The measure of the right angle is 90 degrees so if you know the measure of one of the other angles you can figure out the measure of the third angle. What postulate of theorem allows you to do that?

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The Answer is the Angle Addition Postulate

This postulate says the measure of the whole is equal to the sum of the measures of its non overlapping parts. So if you know the measurements of the right angle and the other angle, then you can subtract those from the sum of all angles to find the last.

For example, say these 3 angles added up to 180 degrees. You know there is a 90 degree angle and say the other angle is 30 degrees. We can subtract the two from the whole to find the third. 180-90-30=60. So the last angle would be the 60 degrees.

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