导数是y=cos (π/4—x)(-1)=-cos (π/4—x)
y=sin(π/4—x)y'=cos(π/4—x)*(π/4-x)' 必须有后面一项，因为是复合函数的求导。=cos(π/4-x)(-1)=-cos(π/4-x).
y = sin(π/4—x)y' = -cos(π/4—x)
y'=-cos(π/4—x)
y'=-cos(π/4-x)
