Section 4.7 [ATTACH=full]1123[/ATTACH] Question 52 cos (arcsin(4/5)) Let u = arcsin(4/5) This leads to sin u = 4/5. The range of the arcsin function is [-pi/2 to pi/2] and sin u is positive. We are in quadrant 1. I know that sin u = opp/hyp. I drew a right triangle in quadrant 1 to find the adjacent side. According to a^2 + b^2 = c^2, the adj side = 3. The original problem, after the u-substitution became cos u = adj/hyp. In this case, cos u = 3/5. So, cos (arcsin(4/5)) = 3/5. You say?