∂z/∂x
=1/(1+y^2/x^2)*(y/x)'
=x^2/(x^2+y^2)*y*(-1/x^2)
=-y/(x^2+y^2)
∂z/∂y
=1/(1+y^2/x^2)*(y/x)'
=x^2/(x^2+y^2)*(1/x)
=x/(x^2+y^2)求函数f（x）=2x^3-6x^2-18x-7的单调区间和极值