若cos2α/sin（α-π/4）=（cos^2a-sin^2a）/[√2/2(sina-cosa)]=(cosa-sina)(cosa+sina)/[√2/2(sina-cosa)]=-√2(cosa+sina)=-√2/2所以cosα+sinα=1/2
若cos2α/sin（α-π/4）=（cos^2a-sin^2a）/[√2/2(sina-cosa)]=(cosa-sina)(cosa+sina)/[√2/2(sina-cosa)]=-√2(cosa+sina)=-√2/2所以cosα+sinα=1/2
