alignequationsline-breaking
I am writing an equation consisting of many lines, however, one particular line is very long and I want to break into a new line.
For example:
\begin{equation}
\begin{aligned}
& \text{minimise} && COST = A + B \\
& \text{subject to} && A = A_1 + A_2 \\
& && B = B_1 + B_2 + B_3 + B_4 + B_5 + B_6 + B_7 \\ % this line is too long
\end{aligned}
\end{equation}
Based on above example, I want to add a line break into the last line.
The following example uses aligned to align the second and right line to the right side.
\documentclass{article}
\usepackage{amsmath}
\newcommand*{\epsO}{\epsilon_0}
\begin{document}
\begin{align*}
\hat{H} \Psi(r,\theta) & = \left\lbrace\frac{-\hbar^2}{2m_er^2} \left[
\frac{\partial}{\partial r} \left(r^2\frac{\partial }{\partial r}\right)
+ \frac{1}{\sin\theta}\frac{\partial}{\partial \theta}
\left(\sin\theta\frac{\partial}{\partial\theta}\right)
+ \frac{1}{\sin^2\theta}\frac{\partial^2}{\partial\varphi^2}\right]
- \frac{e^2}{4\pi \epsO r} \right\rbrace \Psi(x,\theta) \\
&
\begin{aligned}
{}= \frac{-\hbar^2}{2m_er^2} \left[
\frac{\partial}{\partial r} \left(r^2\frac{\partial }{\partial r}\right)\Psi(x,\theta)
+ \frac{1}{\sin\theta}\frac{\partial}{\partial \theta}
\left(\sin\theta\frac{\partial}{\partial\theta}\right)\Psi(x,\theta)
+ \frac{1}{\sin^2\theta}\frac{\partial^2}{\partial\varphi^2}\Psi(x,\theta)\right]\\
- \frac{e^2}{4\pi \epsO r}\Psi(x,\theta)
\end{aligned} \\
\end{align*}
\end{document}
I suggest you use a split environment, which in contrast to multline may be used a subenvironment of equation. You need to specify an alignment point on each line with & and separate lines with \\. In this case the first line should be move left relative to the others and the package mathtools provides a convenient command for this:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\label{wave kinematic}
\begin{split}
\MoveEqLeft
\frac{\partial N(\sigma;\lambda;\theta;t)}{\partial t}
+ \frac{\partial C_{g,\lambda}N(\sigma;\lambda;\theta;t)}
{\partial \lambda} \\
&+ \cos\phi^{-1} \cdot
\frac{\partial C_{f,\phi}N(\sigma;\lambda;\theta;t)}{\partial
\phi} \\
&+ \frac{\partial C_{f,\theta}N(\sigma;\lambda;\theta;t)}{\partial
\theta}
+ \frac{\partial C_{f,\sigma}N(\sigma;\lambda;\theta;t)}{\partial
\sigma}
= \frac{S(\sigma;\theta;\lambda;\varphi;t)}{\sigma}
\end{split}
\end{equation}
\end{document}
I have split across three lines for clarity. If you wanted to just split in two parts, then multlined (notice the extra d) from the mathtools package would be a simpler solution:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\label{wave kinematic}
\begin{multlined}
\frac{\partial N(\sigma;\lambda;\theta;t)}{\partial t}
+ \frac{\partial C_{g,\lambda}N(\sigma;\lambda;\theta;t)}
{\partial \lambda}
+ \cos\phi^{-1} \cdot
\frac{\partial C_{f,\phi}N(\sigma;\lambda;\theta;t)}{\partial
\phi} \\
+ \frac{\partial C_{f,\theta}N(\sigma;\lambda;\theta;t)}{\partial
\theta}
+ \frac{\partial C_{f,\sigma}N(\sigma;\lambda;\theta;t)}{\partial
\sigma}
= \frac{S(\sigma;\theta;\lambda;\varphi;t)}{\sigma}
\end{multlined}
\end{equation}
\end{document}
All the above works with elsarticle class in your updated question. E.g. the first version becomes:
\documentclass[authoryear,preprint,review,12pt]{elsarticle}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\label{wave kinematic}
\begin{split}
\MoveEqLeft
\frac{\partial N(\sigma;\lambda;\theta;t)}{\partial t}
+ \frac{\partial C_{g,\lambda}N(\sigma;\lambda;\theta;t)}
{\partial \lambda} \\
&+ \cos\phi^{-1} \cdot
\frac{\partial C_{f,\phi}N(\sigma;\lambda;\theta;t)}{\partial
\phi} \\
&+ \frac{\partial C_{f,\theta}N(\sigma;\lambda;\theta;t)}{\partial
\theta}
+ \frac{\partial C_{f,\sigma}N(\sigma;\lambda;\theta;t)}{\partial
\sigma}
= \frac{S(\sigma;\theta;\lambda;\varphi;t)}{\sigma}
\end{split}
\end{equation}
\end{document}
Best Answer
If it is just breaking, you can use
multlinedfrommathtools(which is an extended form ofamsmath)