sin^(-1)(x) + cos^(-1)(x) = pi/2 for ALL x in [-1, 1]. tan^(-1)(x) + cot^(-1)(x) = pi/2 for ALL x in R. sec^(-1)(x) + csc^(-1)(x) = pi/2 for |x| >= 1. These are the most useful identities — they allow converting between inverse co-functions instantly. If asked for cos^(-1)(x), you can compute pi/2 - sin^(-1)(x).