so we draw a very steep right triangle with the opposite = 8 and the adjacent = 1. The small sharp angle at the top is the complement of q So "COtangent" is an abbreviation for "COmplement's tangent", and cot(p/2-q) IS q's COmplement's tangent. That's why both are equal. The opposite side of q is the adjacent side of p/2-q and vice-versa. Edwin