dz=f1'dx²+f2'dy/z
=2xf1'dx+f2'(zdy-ydz)/z²
dz+y/z²f2'dz=2xf1'dx+f2'/z dy
dz=(2xf1'dx+f2'/z dy)/(1+y/z²f2')
z=f(x^2,y/z) 求dz
dz=f1'dx²+f2'dy/z
=2xf1'dx+f2'(zdy-ydz)/z²
dz+y/z²f2'dz=2xf1'dx+f2'/z dy
dz=(2xf1'dx+f2'/z dy)/(1+y/z²f2')