1

I don't know what the problem is. Here is the code:

\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
Some text.
\begin{align}
\label{li2}
\text{Let $s \in S$ and $C \in \mathbb{R}$. Let's show $Cs \in S$:} \nonumber \\
\sum_{k=0}^n{a_k D^k(Cs)} &= \sum_{k=0}^n{Ca_k D^ks} \tag{by linearity of $D^k$} \nonumber \\
&= C\sum_{k=0}^n{a_k D^ks} = 0 
\end{align}
\end{document}

enter image description here

2
  • Welcome to TeX.SX! Could you please add the documentclass and required packages, so that it is compilable?
    – TeXnician
    Apr 17, 2018 at 4:25
  • the output seems to directly match your input, your first line is in the first cell of the first row so the alignment point for the = on the next row is at the end of that text, where did you expect the alignment to be? Apr 17, 2018 at 7:00

2 Answers 2

4

There is no reason for making the text part of the alignment. Also, the textual explanation has no relation with the equation number that should refer to both lines.

\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{lipsum}% just for the example

\begin{document}

\lipsum[3]

Let $s \in S$ and $C \in \mathbb{R}$. Let's show $Cs \in S$:
\begin{equation}\label{li2}
\begin{aligned}
\sum_{k=0}^n{a_k D^k(Cs)} &= \sum_{k=0}^n{Ca_k D^ks}
  &&\text{(by linearity of $D^k$)} \\
&= C\sum_{k=0}^n{a_k D^ks} = 0 
\end{aligned}
\end{equation}
\lipsum[4]

\end{document}

enter image description here

If you use \begin{aligned}[b], the equation number will be aligned with the bottom line.

enter image description here

1
  • Thank you for your response, but the problem is that there was a short sentence right before the text, and the succession of the two sentence seperate by a new line is wierd-looking, because both of them are hanging indented, and give the impression that they are together. The second line goes with the equation and not with the text.
    – Jean-Paul
    Apr 18, 2018 at 11:29
2

Welcome to TeX.SE! There were some issues. I don't think it is a good idea to make the first line part of the equation. Then align aligns stuff at some anchor(s), represented by & symbols. Having said this, I am wondering if this closer to what you want.

\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
Let $s \in S$ and $C \in \mathbb{R}$. Let's show $Cs \in S$:
\begin{align}
\sum_{k=0}^n{a_k D^k(Cs)} &= \sum_{k=0}^n{Ca_k D^ks} \tag{by linearity of $D^k$} \nonumber \\
&= C\sum_{k=0}^n{a_k D^ks} = 0 
\label{li2} %<- there is no point having label and \nonumber in the same line
\end{align}
\end{document}

enter image description here

3
  • The problem was that the "Let s \in S..." part had not to be part of the preceding text. I've just solved it using \intertext, but the vertical-spacing isn't ideal. Moreover, I'm wondering if the \tag trick is really a good one or wether there is a more proper way to do it.
    – Jean-Paul
    Apr 17, 2018 at 4:02
  • 2
    @Jean-Paul - What's wrong with \tag? Its purpose is to place material that doesn't fall into the equation number category. For sure, it's not some kind of shady "trick".
    – Mico
    Apr 17, 2018 at 5:10
  • 2
    @Jean-Paul Why should the text Let $s\in S$... be part of the alignment? It is text and should be treated as such.
    – egreg
    Apr 17, 2018 at 8:29

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