题目可能有错!
应该为：
cot(x/y)*sec²x/y*(-x/y²)=-2x/y²csc2x/y
cot(x/y)*sec²x/y*(-x/y²)=(-x/y²)*(cos(x/y)/sin(x/y))*(1/(cos(x/y))^2)
=(-x/y²)/(sin(x/y)*cos(x/y))
(-2x/y²)*csc2x/y=(-2x/y²)*(1/(sin(2x/y))
=(-2x/y²)*(1/(2sin(x/y)*cos(x/y))==(-x/y²)/(sin(x/y)*cos(x/y))