z=㏑tan(x/y)的偏函数
Zx=[1/tan(x/y)]*1/[1+(x/y)2]*(1/y)=[y/(x2+y2)]*[1/tan(x/y)]
Zy=[1/tan(x/y)]*[1/[1+(x/y)2]*(-x/y2)=[-x/(x2+y2)]*[1/tan(x/y)]
Zx=[1/tan(x/y)]*1/[1+(x/y)2]*(1/y)=[y/(x2+y2)]*[1/tan(x/y)]
Zy=[1/tan(x/y)]*[1/[1+(x/y)2]*(-x/y2)=[-x/(x2+y2)]*[1/tan(x/y)]