Latex

Multiline example (amsmath package)

\begin{multline}

\biggl(\sum_{\,i\in\mathbf{n}}a_{l _i}x_i\biggr) \det\mathbf{K}

(t=1,x_1,\dots,x_n;l |l )\\

=\biggl(\prod_{\,i\in\mathbf{n}}\hat x_i\biggr)

\sum_{I\subseteq\mathbf{n}-\{l \}}

(-1)^{\envert{I}}\per\mathbf{A}^{(\lambda)}(I|I)

\det\mathbf{A}^{(\lambda)}

(\overline I\cup\{l \}|\overline I\cup\{l \}).

\label{sum-ali}

\end{multline}

Equation and split example (amsmath package, see attachment)

\begin{equation}

\begin{split}

\framebox{BOX1} & = \framebox{BOX2} \\ & = \framebox{BOX3}

\end{split}

\end{equation}

Align for simple equation array with one number

\begin{align}

& \lambda_1^2 - \lambda_2^2 + (k_1+k_2+k_3)( \lambda_1 - \lambda_2) = 0 \nonumber \\

& \Rightarrow k_1+k_2+k_3 = -(\lambda_1 + \lambda_2)

\end{align}