sin2a=-8/9cos2a=+ -根号17/9(sina+cosa)^2=1+sin2a=1/9然后根据(sin2a)^2+(cos2a)^2=1你需要判断2a的范围cosα+sinα=-1/3=根号2sin(a+π/4)=-1/3α应该是在（3π/4,π）之间,那2α就是（3π/2,2π）之间那么cos2a>...