y=sinx-cosx=sinxcos(π/4)-cosxsin(π/4)=sin(x-π/4)
y=sinx+cosx=sinxcos(π/4)+cosxsin(π/4)=sin(x+π/4)
函数y=sinx-cosx的图象可以看成是由函数y=sinx+cosx的图象像由平移π/2得到的