Images of 完全微分
\begin{eqnarray*} &&\frac{\partial P}{\partial y}=\frac{\partial Q}{\partial x}\\\\ \Leftrightarrow \quad&& \frac{\partial \lambda }{\partial y }(y^2 -xy)+ \lambda(2y-x)= \frac{\partial \lambda }{\partial x}(x^2-2xy)+\lambda(2x-2y)\\\\ \Leftrightarrow \quad&& \frac{\partial \lambda}{\partial y}(y^2 -xy)- \frac{\partial \lambda}{\partial x}(x^2-2xy) =\lambda(3x-4y) \end{eqnarray*}
\begin{eqnarray*} &&\frac{\partial \lambda (x)}{\partial x}Q=\lambda(P_y -Q_x)\\\\ \Leftrightarrow \quad&& \frac{1}{\lambda}\frac{d\lambda (x)}{dx}=\frac{P_y-Q_x}{Q} \end{eqnarray*}
\begin{eqnarray*} &&(1)\quad y\,dx-x\,dy=0\\ &&(2)\quad y^2\,dx-(3x^2+xy)\,dy=0\\ &&(3)\quad (y^2-xy)dx+(x^2-2xy)dy=0 \end{eqnarray*}
\begin{eqnarray*} &&\frac{\partial P}{\partial y}=\frac{\partial Q}{\partial x}\\\\ \Leftrightarrow \quad&& \frac{\partial \lambda}{\partial y}\cdot y + \lambda = \frac{\partial \lambda}{\partial x}\cdot(-x)-\lambda\\\\ \Leftrightarrow \quad&& \frac{\partial \lambda}{\partial y}\cdot y + \frac{\partial \lambda}{\partial x}\cdot x=-2\lambda \end{eqnarray*}
\begin{eqnarray*} &&-\frac{y}{x^2}\,dx+\frac{1}{x}\,dy=0\\ \Leftrightarrow \quad&&d\left(\frac{y}{x}\right)=0\\ \Leftrightarrow \quad&&\frac{y}{x}=C\quad\blacksquare \end{eqnarray*}