Environments

There are 2 environments to display one-line equations.

equation:-

Equations in this environment are numbered.

\begin{equation}
 x + iy
\end{equation}

x + iy (1)

displaymath:-

These won't be numbered. \[, \] can be used as abbreviations for \begin{displaymath} and \end{displaymath}.

\begin{displaymath}
 x + iy
\end{displaymath}

x + iy

Never leave a blank line before these equations; it starts a new paragraph and looks ugly. '\displaystyle' is the font type used to print maths in these display environments. Other relevant environments are:-

math:-

For use in text. \( and \) can be used to delimit the environment, as can the TEX constructions $ and $ . For example, $x=y^2$ gives x=y².

eqnarray:-

This is like a 3 column tabular environment. Each line by default is numbered. You can use the eqnarray* variant to suppress numbering altogether.

\begin{eqnarray}
a1 & = & b1 + c1\nonumber\\
a2 & = & b2 - c2
\end{eqnarray}

a1 = b1 + c1

a2 = b2 - c2 (2)

Maths in these 2 sorts of environments have different default sizes for some characters and other behavioural differences so that a line of maths won't impinge on text lines below or above. If you want to put some non-maths text in amongst maths then enclose it in an \mbox{...}.