cos(2a) = -（√2/2）sin(a-(π/4))
(cos²a-sin²a)= -(√2/2)[sinacos(π/4)-cosasin(π/4)]
(cosa+sina)(cosa-sina)= -(√2/2)[(√2/2)sina-(√2/2)cosa]
(cosa+sina)(cosa-sina) = -(1/2)(sina-cosa)
(cosa+sina)(cosa-sina)+(1/2)(sina-cosa) = 0
(cosa+sina+1/2)(cosa-sina)=0
所以：cosa+sina = -1/2，或者cosa = sina
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