sina+cosa=1/5并且(sina)^2+(cosa)^2=1根据4象限角~算出sina=-3/5 cosa=4/5所以sina*cosa =-12/25sina^3+cosa^3 =39/125sina-cosa=-7/5sina^4-cosa^4=sina^2-cosa^2=-7/25tana=sina/cosa=-3/4