Y=SINx-COSx+SINxCOSx+2
=(SINx-COSx)+SINxCOSx+1/2(sin^2x+cos^2x)+5/2
=(SINx-COSx)-1/2(sinx-cosx)^2+5/2
假设t=sinx-cosx=√2sin(x-π/4）
-π/4