The expression cos (pi/2 − theta) captures a fundamental relationship between cosine and sine. Geometrically, this comes from looking at a right triangle. If theta is one acute angle, then pi/2 − theta is the other acute angle, since the two must add up to a right angle. The cosine of an angle is defined as the ratio of the adjacent side to the hypotenuse, while the sine of the complementary angle uses the opposite side over the same hypotenuse. Because the side opposite one angle is the adjacen