若sin(a减4分之派)分之cos2a=负2分之根号2,则sina加cosa的值为?急

即(cos²a-sin²a)/(sinacosπ/4-cosxsinπ/4)
=(cosa+sina)(cosa-sina)/[√2/2(sina-cosx)
=-(cosa+sina)/(√2/2)
=-√2/2
sina+cosa=1/2