amsmath

name	amsmath
description	LaTeX amsmath/amssymb/mathtools packages for mathematical typesetting. Use when helping users write equations, align math, use mathematical symbols, matrices, theorems, or any advanced math formatting.

amsmath + amssymb + mathtools — Math Typesetting

CTAN: https://ctan.org/pkg/amsmath | https://ctan.org/pkg/mathtools
Manual: texdoc amsmath, texdoc mathtools

Setup

\usepackage{amsmath}    % core math environments
\usepackage{amssymb}    % extra symbols (loads amsfonts)
\usepackage{mathtools}  % fixes + extensions for amsmath (loads amsmath)
\usepackage{amsthm}     % theorem environments

% mathtools loads amsmath, so you can just use:
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{amsthm}

Equation Environments

Single Equations

% Numbered
\begin{equation}
  E = mc^2
  \label{eq:einstein}
\end{equation}

% Unnumbered
\begin{equation*}
  E = mc^2
\end{equation*}
% or: \[ E = mc^2 \]

align — Multiple Aligned Equations

\begin{align}
  f(x) &= x^2 + 2x + 1 \label{eq:f} \\
       &= (x+1)^2       \label{eq:f2}
\end{align}

% Unnumbered: align*
% Suppress single number: \nonumber or \notag before \\

gather — Centered, No Alignment

\begin{gather}
  x + y = 1 \\
  x - y = 3
\end{gather}

multline — Long Single Equation

\begin{multline}
  f(x) = a_0 + a_1 x + a_2 x^2 + a_3 x^3 \\
    + a_4 x^4 + a_5 x^5 + a_6 x^6
\end{multline}

First line left-aligned, last line right-aligned, middle centered.

split — Sub-alignment Inside equation

\begin{equation}
\begin{split}
  f(x) &= a + b + c \\
       &\quad + d + e
\end{split}
\end{equation}