Math that sits in a sentence (or alone)

LaTeX treats mathematics as a separate mode with its own fonts and spacing rules. Every variable, formula, and symbol has to live inside math mode, whether it is a lone n in a sentence or a full equation. There are two kinds: inline math flows within a paragraph, and display math sits centered on its own line, with or without a number.

Inline: $E=mc^2$ or \(E=mc^2\)

Display (unnumbered):
\[
 \int_0^1 x^2 \, dx = \frac{1}{3}
\]

Display (numbered):
\begin{equation}
 \label{eq:energy}
 E = mc^2
\end{equation}

Inline math

A pair of dollar signs, as in $E=mc^2$, sets the formula at text size inside the line. The letters come out in math italic and the spacing around the equals sign is handled for you. \(E=mc^2\) is the equivalent LaTeX-native form; it produces identical output, and because its opening and closing delimiters differ, an unclosed one gives a clearer error message. Use inline math even for a single variable name, so the n in your prose matches the n in your equations.

Display math

\[ ... \] places a formula on its own centered line without a number. It is the right choice for one-off equations you never refer back to. Avoid the old $$ ... $$ form; it is plain TeX rather than LaTeX and produces slightly wrong vertical spacing in some situations.

Numbered equations

The equation environment displays the formula with a number at the margin. \label{eq:energy} attaches a name to that number, and elsewhere you write Equation~\ref{eq:energy} or, with amsmath loaded, \eqref{eq:energy}, which adds the parentheses itself. The ~ is a non-breaking space, so the word and the number stay on one line. References resolve on the second compile; if you see ?? in the output, see question marks instead of numbers.

The packages to load

\usepackage{amsmath,amssymb,amsfonts}

Nearly every mathematical document loads these three. amsmath provides the environments and commands most of these lessons assume, including align and \eqref. amssymb and amsfonts add symbol fonts and extra alphabets such as \mathbb. Put the line in the preamble once and forget about it.

The classic beginner error is math syntax outside math mode: an underscore in plain text stops the compile with “Missing $ inserted”, which has its own lesson. To get a feel for the two modes, type a sentence with $x_i$ inline and the same formula inside \[ ... \] in the live playground and compare how each sits on the page.