Latex: problems with equation numbers and space after the equal-sign

I have a set of really easy formulas/equations and need to present them in the right way. And, as always, I have some trouble with the different possibilities from Latex.

My first attempt was:

\usepackage[fleqn]{amsmath}
\begin{document}
\begin{align}
\label{eq_convert}
p(x) &=& C \cdot x ^ {- \alpha} \nonumber \\
ln(p(x)) &=& ln(C \cdot x ^ {- \alpha}) \nonumber \\
&=& ln(C)+ln(x ^ {- \alpha}) \nonumber \\
ln(p(x)) &=& ln(C) + ln(x) \cdot {- \alpha}
\end{align}
\end{document}

It has only one number for the equation on the bottom, but the equations have to much space to the equal-sign and they are right-aligned after the equal-sign. The output as an image: Thus, I googled my problem and found a new solution with the \equation function and I know, that this is not the best way:

\usepackage[fleqn]{amsmath}
\begin{document}
\begin{equation}
\begin{split}
p(x) &=& C \cdot x ^ {- \alpha} \\
ln(p(x)) &=& ln(C \cdot x ^ {- \alpha})  \\
&=& ln(C)+ln(x ^ {- \alpha}) \\
ln(p(x)) &=& ln(C) + ln(x) \cdot {- \alpha}
\end{split}
\end{equation}
\end{document}

The result looks much better, but I want the counting-number on the bottom. [ I asked this question first in Stack Overflow, but don't get an answer (probably I'm too impatient), anyway, I think this is the right place to ask it. Any ideas how to solve this problem? Thanks a lot.

• Welcome to TEX.SE. First suggestion: isn't $ln$ but $\ln$. – Sebastiano May 29 '18 at 8:47
• Just replace &=& (the old eqnarray syntax) by &=. And to ahve the numbering at the bottom use \begin{aligned}[b] instead of split. – campa May 29 '18 at 8:48
• the syntax is &= not &=& – David Carlisle May 29 '18 at 8:48
• Add \usepackage[fleqn,tbtags]{amsmath} into your preamble. – Mishra S May 29 '18 at 8:56

In addition to fixing the syntax error by changing the four instances of &=& to &=, you may also wish to (a) switch from an align environment to a nested pair of equation and aligned environments -- to get rid of the need for all those \nonumber directives -- and (b) write \ln rather than just ln and also use fewer parentheses to declutter the "look" of the equations. \documentclass{article} % choose a suitable document class
\usepackage[fleqn]{amsmath}
\begin{document}
\begin{equation} \label{eq_convert}
\begin{aligned}[b] % "[b]" for bottom alignment of eqn. number
p(x) &= C x ^ {- \alpha} \\
\ln p(x) &= \ln(C x ^ {- \alpha}) \\
&= \ln C +\ln(x ^ {- \alpha})  \\
&= \ln C - \alpha\ln x
\end{aligned}
\end{equation}
\end{document}