y=x²f(1/x)dy=[2xf(1/x)+x²df(1/x)/d(1/x)·d(1/x)/dx]dx=2xf(1/x)dx-[df(1/x)/d(1/x)]dx(如y=sin(1/x) dy/dx=d[sin(1/x)/d(1/x)]·d(1/x)/dx=cos(1/x)·(-1/x²)=-cos(1/x)/x²)
