# Line up equations

I want to line up my two equations by the two first equal signals so I wrote this

\begin{align}
\left\
\begin{matrix}
\Delta l &= \Delta l_1 + \Delta l_2\\
N &= N_1(\Delta l_1)=N_2(\Delta_2)\\
\end{matrix}\right.
\end{align}


But I'm getting this: • Welcome to TeX.SE. While code snippets are useful in explanations, it is always best to compose a fully compilable MWE that reproduces the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. – Peter Grill Oct 28 '14 at 2:31

You need to use an inner math environment along with cases In this case you don't really need to use align and simply equation would yield identical results as above

## Notes:

• As what you have clearly is not a matrix you should not use the matrix environment.

## Code: align

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
\begin{cases}
\begin{aligned}
\Delta l &= \Delta l_1 + \Delta l_2\\
N &= N_1(\Delta l_1)=N_2(\Delta_2)\\
\end{aligned}
\end{cases}
\end{align}
\end{document}


## Code: equation:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{equation}
\begin{cases}
\begin{aligned}
\Delta l &= \Delta l_1 + \Delta l_2\\
N &= N_1(\Delta l_1)=N_2(\Delta_2)\\
\end{aligned}
\end{cases}
\end{equation}
\end{document}

• What's the reason for the aligned environment inside the cases environment? – Bernard Oct 28 '14 at 14:15
• @Bernard -- the question asked for the two equals signs to be aligned. cases doesn't ordinarily do that; it aligns the lines at the beginning. – barbara beeton Oct 28 '14 at 15:23

If you like matrix-like tools, maybe array will be suitable for you.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

$\left\{ \begin{array}{@{}r@{\;=\;}l} \Delta l & \Delta l_1 + \Delta l_2\\ N & N_1(\Delta l_1)=N_2(\Delta_2)\\ \end{array} \right.$

\end{document} @ kills \arraycolsep, adding its argument instead.

I have another idea, maybe split will be suitable for you.

 \documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\left\{
\begin{split}
\Delta l &= \Delta l_1 + \Delta l_2\\
N &= N_1(\Delta l_1)=N_2(\Delta_2)\\
\end{split}\right.
\end{equation}
\end{document}


One can have a slightly simpler syntax with the empheq package (which loads mmathtools, hence amsmath):

\documentclass{article}

\begin{document}

\begin{equation}[left=\empheqlbrace]
\!\begin{aligned}
Δ l &= Δ l_1 + Δ l_2\\
N &= N_1(Δ l_1)=N_2(Δ_2)
\end{aligned}
\end{equation}

\end{document} • Why empheq? Just \left\{\begin{aligned}...\end{aligned}\right. is sufficient. – egreg Oct 28 '14 at 18:59
• @egreg: I think that it's more expressive and (slightly) simpler to use the optional argument to the equation environment, that's why. – Bernard Oct 28 '14 at 19:08