sin（π/4-x)为什么等于cos(x+π/4)

sin(π/4-x)=sin(π/4)cosx-cos(π/4)sinx=（√2/2）cosx-（√2/2）sinx;
cos(x+π/4)=cosxcos(π/4)-sinxsin(π/4)=（√2/2）cosx-（√2/2）sinx；