Complementary angles add up to 90°. Angles 1 and 3 are complementary, so the formula is ∠1 + ∠3 = 90°. Given that Angle 1 equals to angle 2, we substitute angle 2 for angle one in the equation stated before. Now we have ∠2 + ∠3 = 90°, so angles 2 and 3 are complementary
