# How to write two equations as part of one equation?

\begin{align} \mathbf{x_{t}=f_{t}(x_{t-1},u_{t})}\\ \mathbf{y_{t}=g_{t}(x_{t},v_{t})} \label{eq:state-space&obs-equ} \end{align}


I want that y(t) equation should come on next line but it comes on same line? how to solve this problem?

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Use \mathbf{x_{t}}&=\mathbf{f_{t}(x_{t-1},u_{t})}\\ \mathbf{y_{t}}&=\mathbf{g_{t}(x_{t},v_{t})} –  Sigur Jul 28 '14 at 18:34
I already done this, but equations come on one line –  zurr Jul 28 '14 at 18:36
You can not nest equation and align. Use only align. –  Sigur Jul 28 '14 at 18:36
Ok, I do not nest them, but when I use \begin{align} and \end{align} there comes some error –  zurr Jul 28 '14 at 18:39
Please, note that you have to use \mathbf twice since the use of &=. Correct the braces {}. –  Sigur Jul 28 '14 at 18:43

\documentclass{article}
\usepackage{mathtools}
\begin{document}

\boldmath
\begin{align}
\begin{aligned}
x_t &= f_t(x_{t-1},u_t)\\
y_t &= g_t(x_t,v_t)
\end{aligned}\label{eq:state-space&obs-equ}
\end{align}

\unboldmath
\begin{align}
x_t &= f_t(x_{t-1},u_t)   &  y_t &= g_t(x_t,v_t)
\end{align}

\end{document}


However, it is not a good idea to set the complete equations in bold.

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still error, i am using winedit –  zurr Jul 28 '14 at 18:42
and I want to be equation 1 and not equation 1 &2 –  zurr Jul 28 '14 at 18:46
see my edited answer. –  Herbert Jul 28 '14 at 18:49
\documentclass[border=12pt,preview]{standalone} % change it back to your document class
\usepackage{mathtools}

\begin{document}
\section*{side-by-side}
\begin{align}
x_{t} &= f_{t}(x_{t-1},u_{t}) & y_{t} &=g_{t}(x_{t},v_{t}) \label{eq:label1}
\end{align}

\section*{split with single number}
$$\begin{split} x_{t} &= f_{t}(x_{t-1},u_{t}) \\ y_{t} &=g_{t}(x_{t},v_{t}) \end{split}\label{eq:label2}$$

\section*{aligned with single number}
\! \begin{aligned} x_{t} &= f_{t}(x_{t-1},u_{t}) \\ y_{t} &=g_{t}(x_{t},v_{t}) \end{aligned}\label{eq:label3}
\end{document}


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The problem is simply that you can't use align inside equation (and LaTeX will show you an error message that should tell you something is wrong with this). Just delete the and lines. This should work fine:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\mathbf{x_{t}=f_{t}(x_{t-1},u_{t})}\\
\mathbf{y_{t}=g_{t}(x_{t},v_{t})}
\label{eq:state-space&obs-equ}
\end{align}
\end{document}


There are other things you may want to fix about this, e.g. putting in alignment points with &, but the minimum change required to fix the problem you're having now is getting rid of that equation environment.

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