Aligning equations in LaTeX

The amsmath package provides the align environment for aligning multiple equations at a common point (typically the = sign). Use & to mark the alignment point.

The align environment

\begin{align}
    x + y &= 5 \\
    2x - y &= 1
\end{align}

% Sans numérotation : align*
\begin{align*}
    (a + b)^2 &= a^2 + 2ab + b^2 \\
    (a - b)^2 &= a^2 - 2ab + b^2
\end{align*}

Multi-line calculation

To develop a calculation step by step, align on the = sign:

\begin{align*}
    f(x) &= (x+1)(x-2) \\
         &= x^2 - 2x + x - 2 \\
         &= x^2 - x - 2
\end{align*}

Centered equations with gather

If you simply want to center multiple equations without aligning them, use gather:

\begin{gather*}
    E = mc^2 \\
    F = ma \\
    PV = nRT
\end{gather*}