$\left\{ \begin{array}{l} 4x-7y+c=0\\ x+3y-14c=0 \end{array} \right.$$\Leftrightarrow \left\{ \begin{array}{l} x=14c-3y\\ 4(14c-3y)-7y+c=0 \end{array} \right.$$\Leftrightarrow \left\{ \begin{array}{l} x=14c-3y\\ 19y=57c\end{array} \right.$$\Leftrightarrow \left\{ \begin{array}{l} y=3c\\ x=5c\end{array} \right.$la nghiem cua he
khhi do :$\frac{x^{4}+x^{3}y}{y^{4}+c^{4}}=\frac{(5c)^{4}+(5c)^{3}(3c)}{(3c)^{4}+c^{4}}=12,2$