LaTeX数学公式排版

\begin{aligned}(modm)
    x & = a + & b \\
    c + d & = & y \\
\end{aligned}

A = 
\left[
    \begin{matrix}
        a_1 & a_2 \\
        b_1 & b_2 \\
    \end{matrix}
\right]

\begin{cases}
    a_{11} x_1 + a_{12} x_2 & = b_1 \\
    b_2 & = a_{21} x_1 + a_{22} x_2 \\
\end{cases}