[Tex/LaTex] Align, equation numbering

alignsubequations

I try to get a equation like this:
enter image description here

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:
enter image description here

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

Best Answer

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.

equation

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