This is a case where visual formatting seems necessary, so it should be dealt with only in the very last stage of document preparation.

You can pretend that the very last part of the formula “is not there”:

```
\documentclass[12pt, oneside]{scrbook}
\usepackage{amsmath,mathtools}
\DeclareMathOperator{\E}{E}
\newcommand{\with}{\,;}
\newcommand{\dx}{\mathop{}\!\mathrm{d}}
\newcommand{\expect}[2][]{\E\if\relax\detokenize{#1}\relax\else_{#1}\fi \left[#2\right]}
\begin{document}
\begin{equation}
\begin{aligned}
\mu_{D_-}
&= \expect{f'(S_-) \cdot \sum_{i=1}^{N} W D} \\
&= N \mu_W \mu_D \int_{-\infty}^{\infty} f'(s) p_\mathcal{N}(s \with \mu_{S_-}, \sigma_{S_-}) \dx{s} \\
\sigma_{D_-}^2
&= \expect{f'(S_-)^2 \cdot \left(\sum_{i=1}^{N} W D\right)^2} - \mu_{D_-}^2 \\
&= N(\sigma_W^2 \sigma_D^2 + \mu_W^2 \sigma_D^2 + \sigma_W^2 \mu_D^2 + N \mu_W^2 \mu_D^2)
\int_{-\infty}^{\infty} f'(s)^2 p_\mathcal{N}(s \with \mu_{S_-}, \sigma_{S_-}) \dx{s}
\mathrlap{{} - \mu_{D_-}^2 \,.}
\end{aligned}
\end{equation}
\end{document}
```

I amended your definition of `\dx`

, because it would give wrong spacing on the right. Also `\;`

seems too big for `\with`

(to be honest, I wouldn't add a space before the semicolon).

Your definition of `\expect`

with automatic `\left`

and `\right`

produces too big delimiters in many cases (see the third line, for instance).

Here's an amended version that uses `xparse`

and `mathtools`

features; the syntax for `\expect`

is

```
\expect{formula}
\expect[\big]{formula}
\expect*{formula}
\expect_{sub}{formula}
\expect_{sub}[\big]{formula}
\expect_{sub}*{formula}
```

as shown in the added equation. In the optional argument one can use `\big`

, `\Big`

, `\bigg`

or `\Bigg`

; the *-version uses `\left`

and `\right`

.

Note also the addition of `\,`

and `\!`

in a few places.

I suggest to use `subequations`

, together with a new command:

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\newcommand{\subtag}[1]{\tag{\theparentequation#1}}
Here is first a single equation
\begin{subequations}
\begin{equation}
a = b, \qquad b = c \subtag{a,b}
\end{equation}
and then an alignment
\begin{gather}
\begin{align}
a &= b, & b = c \subtag{c,d}\\
d &= f = h, & f = g \subtag{e,f}
\end{align}\\
g = h \subtag{g}
\end{gather}
\end{subequations}
\end{document}
```

## Best Answer

The very thing you want is the

`split`

environment, provided by the`amsmath`

class.Sample code.Result.