Recall that the sum of the three interior angles of any triangle is 180 degrees. (This follows from Euclid’s Parallel Postulate.)
Suppose angle 3 is a right angle. Then its measure is 90 degrees and the sum of the measures of angles 1 and 2 must be 90 degrees.
Recall that two angles are complementary if the sum of their measures is 90 degrees.
It follows that the two acute angles of any right triangle are complementary. We will need this fact later.
