Question: what is the relationship between the sine and cosine of complementary angles in a right triangle?
Answer: Since the measures of these acute angles of a right triangle add to 90º, we know these acute angles are complementary. ∠A is the complement of ∠B, and ∠B is the complement of ∠A. The sine of any acute angle is equal to the cosine of its complement. The cosine of any acute angle is equal to the sine of its complement.