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