\left \{ \begin{array}{rcl} ax+by&=&c \\ a'x+b'y&=&c' \end{array} \right.
\begin{cases} x+y &=2 \\ x-3y &=4 \end{cases}
Réponses
$$\left \{
\begin{array}{rcl}
ax+by&=&c \\
a'x+b'y&=&c'
\end{array}
\right.
$$
\begin{cases}
x+y&=2\\
x-3y&=4
\end{cases}
\]