zx=yf1'-y/x²f2'zxx=y²f11''-y²/x²f12''+2y/x³f2'-y²/x²f21''+y²/x^4f22''
z'=f'1(xy,y/x)*y+f'2(xy,y/x)*(-y/x^2)z"=f"11(xy,y/x)*y^2+f"12(xy,y/x)*(-y^2/x^2)+f"21(xy,y/x)*(-y^2/x)+f'(xy,y/x)*(y/x^3)+f"22(xy,y/x)*(y^2/x^2)