How can I align my equations as in the image below?
I know
\begin{equation}
\begin{split}
x_1'(t) &= x_1(t)+2 x_2(t) \\
x_2'(t) &= 3 x_1(t)
\end{split}
\end{equation}
but it will only align the equal signs and not the variables.
You can use
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat*}{3}
x_{1}'(t) &= &x_{1}(t) &{}+{}& 2&x_{2}(t)\\
x_{2}'(t) &= 3&x_{1}(t) & & &
\end{alignat*}
\end{document}
Notice that I have added three alignment points in case you need to vertically align the second factor on the right-hand side too.
I hope it's clear how to extend this to more than three alignment points. (Fore n alignment points, 2n-1 &s are needed.)
P.S. Remember the use of {} to get the correct spacing around the +.
Update
In case you don't need more than two alignment points, you can use the following:
\begin{alignat*}{2}
x_{1}'(t) &={}& &x_{1}(t) + 2x_{2}(t)\\
x_{2}'(t) &={}& 3&x_{1}(t)
\end{alignat*}
x_1(t) needs to align with x_1(2t). In the second example, the extra & makes for better alignment in such cases.
– Dan
Dec 09 '13 at 02:54
Especially if your system of equations is fairly small -- as is the case with the example you've posted -- you could simply use \phantom directives: They insert whitespace equivalent to their arguments. (However, if the system of equations gets larger, it may be worth incurring the overhead associated with the other proposed methods.)
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{split}
x_1'(t) &= \phantom{3}x_1(t)+2 x_2(t) \\
x_2'(t) &= 3 x_1(t)
\end{split}
\end{equation}
\end{document}
Simple array:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
% \begin{split}
\begin{array}{r@{\;}r@{}l}
x_1'(t) =& x_1(t)&{}+2 x_2(t) \\
x_2'(t) =& 3 x_1(t)&
% \end{split}
\end{array}
\end{equation}
\end{document}
Probably vertical spacing needs some correction by adding a suitable length after \\. BTW: one can induce that an image in your example is an effect of just array, but without the correction of \arraycolsep. The spacing around + and = signs is too big.
It is possible to use also systeme.
\documentclass{article}
\usepackage{systeme,mathtools}
\sysdelim..
\newlength\mylen
\settowidth\mylen{$x_2'(t)$}
\begin{document}
\[
\systeme{
x_1'(t) \hspace{-\mylen} = x_1(t)+2 x_2(t),
x_2'(t) = 3 x_1(t)}
\]
\end{document}
\documentclass{...},\begin{document}, and\end{document}. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – jub0bs Dec 08 '13 at 19:05\systemeapproach mentioned in one of the answers in your link won't work in the present case, as it'll insist on placing the variablesx_1'(t)andx_2'(t)in different columns... – Mico Dec 08 '13 at 20:22