3cos2a=sin(π/4-a)3[(cosa)^2-(sina)^2]=√2/2(cosa-sina)(cosa-sina)(3cosa+3sina-√2/2)=0又a∈(π/2,π),所以sina>0 cosa<0所以cosa-sina不等于0所以cosa+sina=√2/6平方 1+2sinacosa=1/18sin2a=-17/18
