Brief Latex notes for equations

In-line: $…$

Single line, without equation number: \[ … \] or \begin{equation*} … \end{equation*}

Single line, with equation number: \begin{equation} … \end{equation}

Multi-line, without equation number: \begin{align*} ... \end{align*}

Multi-line, with equation number: \begin{align} ... \end{align}

"a double backslash (\\) is used to separate the lines, and an ampersand symbol (&) is used to indicate the place at which the formulas should be aligned."

For align, \label must be put in front of each equation. For equation, it does not matter you put it in front or in the end.