Mardi 4 octobre
$$\left \{ \begin{array}{l} 2x - y = 5 \\\\ 5x - 3y = 5 \end{array} \right.$$
$$\left \{ \begin{array}{l} x + y + z = 30 \\\\ x - y + z = 10 \\\\ x + y - z = 0 \end{array} \right.$$
$$\left \{ \begin{array}{l} x + 2y - z = 6 \\\\ x + y + z = 12 \\\\ -x + y + 2z = 11 \end{array} \right.$$