Question

The sine of 58º is equal to the cosine of what angle?

Answer 1

Check the picture below.

in a right-triangle, the cosine of one of the acute angles, is equivalent to the sine of the "other" acute angle, as the example shows in the picture.

and since both acute angles are "complementary angles",  namely they add up to 90°, then

sin(58°)  =  cos(90° - 58°)