Thus, we can conclude that if 2 angles are complementary (see your answer to (1)), then the sine of one angle is equal to the cosine of its complement!
The "CO" in "COsine" literally means "COmplement", or "COmplement of sine"! That is, the sine of one acute angle in a right triangle is the same as the COsine of the other acute angle, its COmplement, in that right triangle!