cos(π/4-α)=1/3cos[π/2-(π/4+α)]=1/3sin（π/4+α）=1/3cos²（3π/4+α）-sin（π/4+α）=cos²[π/2+（π/4+α）]-sin（π/4+α）=sin²（π/4+α）-sin（π/4+α=1/9-1/3=-2/9
