How to Automatically Break Long Equations and Align by Equals in LaTeX
Code
To automatically align equations that grow long, use the breqn package.
dmath
You can place an equation between the opening and closing dmath delimiters; when the equation becomes long it is automatically line-broken and aligned at the equals sign.
\documentclass{article}
\usepackage{breqn}
\begin{document}
\begin{dmath}
dx = -0.018 x^{2} + 0.023 x y - 0.008 y^{2} + 0.033 x^{3} - 0.083 x^{2} y + 0.065 x y^{2} - 0.012 y^{3} + 0.02 x^{4} - 0.093 x^{3} y + 0.169 x^{2} y^{2} - 0.137 x y^{3} + 0.038 y^{4} - 0.037 x^{5} + 0.192 x^{4} y - 0.412 x^{3} y^{2} + 0.447 x^{2} y^{3} - 0.244 x y^{4} + 0.054 y^{5} + 0.011 x^{6} - 0.066 x^{5} y + 0.173 x^{4} y^{2} - 0.241 x^{3} y^{3} + 0.188 x^{2} y^{4} - 0.077 x y^{5} + 0.013 y^{6}
\end{dmath}
\begin{dmath}
dy = 0.009 + 0.006 x - 0.003 y + 0.005 x^{2} - 0.005 y^{2} - 0.025 x^{3} + 0.073 x^{2} y - 0.086 x y^{2} + 0.034 y^{3} - 0.025 x^{4} + 0.109 x^{3} y - 0.157 x^{2} y^{2} + 0.094 x y^{3} - 0.018 y^{4} + 0.068 x^{5} - 0.362 x^{4} y + 0.771 x^{3} y^{2} - 0.82 x^{2} y^{3} + 0.439 x y^{4} - 0.095 y^{5} - 0.027 x^{6} + 0.175 x^{5} y - 0.472 x^{4} y^{2} + 0.677 x^{3} y^{3} - 0.547 x^{2} y^{4} + 0.236 x y^{5} - 0.042 y^{6}
\end{dmath}
\end{document}
dgroup
For systems of equations or ordinary differential equations, you need to align several equals signs, and in that case you can use dgroup. Place several dmath blocks inside the opening and closing dgroup delimiters.
\documentclass{article}
\usepackage{breqn}
\begin{document}
\begin{dgroup*}
\begin{dmath*}
\dot{x} = 5.1747 - 17.932x + 9.5378y - 11.764z + 23.656x^{2} + 6.5559xy + 30.334xz - 23.801y^{2} - 6.3831yz + 7.976z^{2} - 9.7456x^{3} - 9.9892x^{2}y - 18.991x^{2}z + 38.4xy^{2} + 6.9991xyz - 10.694xz^{2} - 19.787y^{3} + 6.3091y^{2}z + 5.0707yz^{2} - 1.9369z^{3}
\end{dmath*}
\begin{dmath*}
\dot{y} = 1.0x - 1.0y + 1.0z
\end{dmath*}
\begin{dmath*}
\dot{z} = -15.0y
\end{dmath*}
\end{dgroup*}
\begin{dgroup*}
\begin{dmath*}
\dot{x} = 5.031 - 17.617x + 9.5957y - 11.543z + 23.464x^{2} + 5.8539xy + 30.084xz - 21.77y^{2} - 6.6587yz + 7.9488z^{2} - 9.727x^{3} - 9.3318x^{2}y - 18.975x^{2}z + 35.698xy^{2} + 7.6657xyz - 10.732xz^{2} - 17.426y^{3} + 5.0784y^{2}z + 5.2821yz^{2} - 1.9521z^{3}
\end{dmath*}
\begin{dmath*}
\dot{y} = 1.0x - 1.0y + 1.0z
\end{dmath*}
\begin{dmath*}
\dot{z} = -15.0y
\end{dmath*}
\end{dgroup*}
\end{document}
