# Align, equation numbering

I try to get a equation like this:

This is my code:

\documentclass{scrartcl}
\usepackage[a4paper]{geometry}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lipsum}
\usepackage{multicol}
\usepackage{ragged2e}
\usepackage{quoting}
\usepackage{microtype}
\usepackage{lmodern}
\usepackage{anyfontsize}
\usepackage{textcomp}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{mathtools}

\begin{document}
\begin{subequations}
\begin{align}
\Delta\,(r, \theta, \varphi) f = \frac{1}{r^{2}}\Bigg[\frac{\partial}{\partial r}\left(r^{2}\frac{\partial f}{\partial r}\right) \nonumber + &\hspace{3cm}\\ \nonumber
& \nonumber + \frac{1}{\sin \theta}\frac{\partial}{\partial \theta}\left(sin\,\theta \frac{\partial f}{\partial \theta}\right) + \frac{1}{\sin ^{2}\theta}\frac{\partial ^{2}f}{\partial \varphi ^{2}}\Bigg]\\
\end{align}
\end{subequations}
\end{document}


And this is my document:

Could someone tell me how to center the quation number 2a for the second row? And how to align the two "+" of the equations?

• The main error is that you have an extra \ line break at the end of the equation. And thus your number is actually centered to an invisible third line. – Yan Zhou Dec 7 '16 at 22:48

Put & to the left of symbols you want to align, and don't put \nonumber in a line that you want numbered. Don't end an align environment with \\ (creates an unwanted extra line), and don't load packages you don't need.

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{subequations}
\begin{align}
\Delta\,(r, \theta, \varphi) f =
\frac{1}{r^{2}}\Bigg[
\frac{\partial}{\partial r}\left(
r^{2}\frac{\partial f}{\partial r}\right) &+{}\nonumber\\
&+ \frac{1}{\sin \theta}\frac{\partial}{\partial \theta}
\left(\sin\,\theta \frac{\partial f}{\partial \theta}\right)
+ \frac{1}{\sin ^{2}\theta}
\frac{\partial ^{2}f}{\partial \varphi ^{2}}\Bigg]
\end{align}
\end{subequations}
\end{document}


Here, you need {} after + at the end of the first line because you get different spacing around these symbols if nothing follows them.

• actually, multline might be more appropriate here than align, since it is treated as a single equation so gets only one number. – barbara beeton Dec 7 '16 at 20:14
• @barbarabeeton --- I agree, but the OP asked for alignment, so I provided it, even though I think it looks a bit odd. – Ian Thompson Dec 7 '16 at 20:22
• +1. For the equation at hand, it so happens that it doesn't matter whether you write \Bigg[ and \Bigg] (making both brackets have type math-ord) or, instead, \Biggl[ and \Biggr] (making the symbols of type math-open and math-close, respectively). In general, though, it's a good idea to use the math-open and math-close versions: it can affect spacing matters advantageously, and it doesn't cost that much extra upfront effort, does it? – Mico Dec 7 '16 at 20:31
• @Mico --- I have to admit that I didn't even look at the delimiter commands. One has to answer quickly in these parts. – Ian Thompson Dec 7 '16 at 20:34

since i suggested using multline in a comment, here is an example using that environment. i've omitted (most) packages that aren't needed for the example.

because the two parts of this example aren't wide enough to fill the page width, they needed space to be added on either side to push them toward the middle. the amount of space used was determined by experimentation.

\documentclass{scrartcl}
\usepackage[a4paper]{geometry}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{mathtools}

\begin{document}
\begin{subequations}
%\begin{align}
\begin{multline}
\hspace*{1.13cm}
\Delta\,(r, \theta, \varphi) f = \frac{1}{r^{2}}\Bigg[\frac{\partial}{\partial r}\left(r^{2}\frac{\partial f}{\partial r}\right) + \\
+ \frac{1}{\sin \theta}\frac{\partial}{\partial \theta}\left(sin\,\theta \frac{\partial f}{\partial \theta}\right) + \frac{1}{\sin ^{2}\theta}\frac{\partial ^{2}f}{\partial \varphi ^{2}}\Bigg]
\hspace{1.13cm}
%\end{align}
\end{multline}
\end{subequations}
\end{document}