1. The problem statement, all variables and given/known data sin-1 (-1/2) = -pi/6 3. The attempt at a solution I was wondering why. Compare to cos-1(-sqrt(2)/2) = 3pi/4, this is done by pi/4 + pi/2 since cos is negative in Q2 AND Q3, this statement is valid, and the arcos is within -pi/2 and pi/2, so my answer is valid. well, i guess? now, back to the problem with sin-1(-1/2) how is it -pi/6? i know Q2 and Q2 are negative sin.... thanks.