# How to split equations in subequation mode

I have the following set of equations in subequations mode and i want to split some of those long expressions to 2-3 lines:

\documentclass[12pt]{report}
\usepackage{amsmath}
\begin{document}
\begin{subequations}
\begin{align}
\frac{\mathrm{d} h}{\mathrm{d} t} &= -\frac{3}{2} \frac{J_2\mu R^2}{r^3} \sin^2i \sin 2u \\
\frac{\mathrm{d} e}{\mathrm{d} t} &= \frac{3}{2} \frac{J_2\mu R^2}{h r^3} \left \{\frac{h^2}{\mu r}\sin \theta \left ( 3\sin^2i \sin 2u-1 \right ) -\sin 2u \sin^2i \left [ \left ( 2+e\cos \theta\right )  \cos\theta +e \right ]\right  \} \\
\frac{\mathrm{d} \theta}{\mathrm{d} t} &=\frac{h}{r^2} +\frac{3}{2} \frac{J_2\mu R^2}{e h r^3} \left \{\frac{h^2}{\mu r}\cos \theta \left ( 3\sin^2i \sin 2u-1 \right ) + \left ( 2+e\cos \theta \right ) \sin 2u \sin^2i \sin \theta \right \} \\
\frac{\mathrm{d} \Omega}{\mathrm{d} t} &= -3 \frac{J_2\mu R^2}{h r^3} \sin^2i \cos i \\
\frac{\mathrm{d} i}{\mathrm{d} t} &= -\frac{3}{4} \frac{J_2\mu R^2}{h r^3} \sin 2u \sin 2i \\
\frac{\mathrm{d} \omega}{\mathrm{d} t} &= \frac{3}{2} \frac{J_2\mu R^2}{e h r^3} \left \{\frac{h^2}{\mu r}\cos \theta \left ( 1-3\sin^2 i \sin^2 u \right )-\left ( 2+e\cos \theta\right )\sin 2u \sin^2i \sin \theta +2e\cos^2 i \sin^2 u\right  \}
\end{align}
\label{J2_gauss}
\end{subequations}
\end{document}


I followed this post but every time that i go to split mode, i encounter an error about matching brackets.

• Welcome to TeX.SE! – Mico Sep 23 '18 at 12:48
• Welcome to TeX.SX! Please don't post code fragments. Instead, put your fragments into a complete compilable document that shows the problem. – BambOo Sep 23 '18 at 12:49
• i want to split the long equations, for example 4.13b, to two lines. in equation 4.13b i want to insert the -\sin 2u \sin^2i[...] to another line. – Esi Sep 23 '18 at 12:53

The reason you can't currently introduce line breaks is that you're overusing \left and \right: These directives must come in pairs, and they are not allowed to be split across lines. I suggest that you (a) get rid of all \left and \right directives -- 7 out of the 10 instances aren't needed anyway and (b) use \Bigl and \Bigr to size the 3 pairs of curly braces. Then it's straightforward to introduce suitable line breaks; don't forget to introduce three \notag directives as well.

\documentclass{article}
\usepackage[margin=1in]{geometry}
\usepackage{amsmath}
\counterwithin{equation}{section}
\DeclareMathOperator{\dee}{d\mskip-1.5mu}
\newcommand\diff[2]{\frac{\dee #1}{\dee #2}}
\begin{document}
\setcounter{section}{4}  % just for this example
\setcounter{equation}{12}

\begin{subequations}
\begin{align}
\diff{h}{t}
&= -\frac{3}{2}\frac{J_2\mu R^2}{r^3}\sin^2 i\sin 2u \\
\diff{e}{t}
&= \frac{3}{2} \frac{J_2\mu R^2}{h r^3} \Bigl\{ \frac{h^2}{\mu r}\sin\theta (3\sin^2i\sin 2u-1) \notag\\
&\qquad -\sin 2u\sin^2i [ (2+e\cos\theta)\cos\theta +e ] \Bigr\} \\
\diff{\theta}{t}
&=\frac{h}{r^2} +\frac{3}{2} \frac{J_2\mu R^2}{e h r^3}
\Bigl\{ \frac{h^2}{\mu r}\cos\theta (3\sin^2i\sin 2u-1) \notag\\
\diff{\Omega}{t}
&= -3 \frac{J_2\mu R^2}{h r^3}\sin^2i\cos i \\
\diff{i}{t}
&= -\frac{3}{4} \frac{J_2\mu R^2}{h r^3}\sin 2u\sin 2i \\
\diff{\omega}{t}
&= \frac{3}{2} \frac{J_2\mu R^2}{e h r^3}
\Bigl\{\frac{h^2}{\mu r}\cos\theta (1-3\sin^2 i\sin^2 u) \notag\\
&\qquad -(2+e\cos\theta)\sin 2u\sin^2i\sin\theta +2e\cos^2 i\sin^2 u \Bigr\}
\end{align}
\label{J2_gauss}
\end{subequations}
\end{document}

• it worked! perfect! :-) – Esi Sep 23 '18 at 13:01
• How is it that the original code works fine with pdflatex ? – BambOo Sep 23 '18 at 13:05
• Don't bother, I just figured what I missed... – BambOo Sep 23 '18 at 13:07
• @BambOo - The original code did not work fine, in the sense that the omnipresent \left and \right directives did not permit line-breaking. – Mico Sep 23 '18 at 13:07
• i just took the subequations part and \newcommand\diff[2]{\frac{\dee #1}{\dee #2}}and inserted to my document. and just worked! – Esi Sep 23 '18 at 13:07