Equations in latex

system of equations

individually numbered

\begin{align
\end{align}

not numbered


\begin{align*}
\end{align*}

single number


\begin{equation}
\begin{aligned}
  dr_t &= \kappa ( \theta - r_t ) dt + \sigma_r r^{\xi} dW_t \\
  dA_t &= \mu A_t dt + \sigma_A^{\alpha} dZ_t,
\end{aligned}
\end{equation}


source:
http://tex.stackexchange.com/questions/13396/how-to-get-only-one-vertically-centered-equation-number-in-align-environment-wit


only last

\documentclass{article}
\usepackage{amsmath}
\newcommand\numberthis{\addtocounter{equation}{1}\tag{\theequation}}
\begin{document}
\begin{align*}
a &=b \\
  &=c \numberthis \label{eqn}
\end{align*}
Equation \eqref{eqn} shows that $a=c$.
\begin{equation}
d = e
\end{equation}
\end{document}

source: http://tex.stackexchange.com/questions/42726/align-but-show-one-equation-number-at-the-end

matrix

\[

\begin{bmatrix}
a_{x,1} b_{y,1} -  a_{y,1} b_{x,1} & 0\\
0                             & a_{x,2} b_{y,2} -  a_{y,2} b_{x,2} \\
\end{bmatrix}=
\]