The problem is... What to put as my answer :P arcsin(Sin(7Pi/5)) http://mathworld.wolfram.com/TrigonometryAnglesPi5.html The solution is -2Pi/5 The problem I'm having is this appears to me as a trick question. Would it be acceptable to say sin(7pi/5) = -(sqrt(5/8+sqrt(5)/8)) make a note of an identity. Then simply say arcsin(-(sqrt(5/8+sqrt(5)/8))) =-2Pi/5? Assuming this is correct, would not all the work I just cut out be acceptable because I'm not trying to prove the identity over again? Comments suggestions would be appreciated. Thanks. PS: Sorry for lack of tex.