# Strange behavior in align environment

I am getting this irregular behavior in horizontal alignment of some splitted equations over my document, this is only one example of it. The problem goes in the second split (which has practically the same structure than the first one). I have tried to correct it by putting \\ here and there but with no success. I know that it could have a simple solution, but I can find it. What can I do? Is it a bug of align environment? \documentclass{article}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath}

\DeclareMathOperator{\di}{d\!}
\DeclareMathOperator{\sen}{\rm sen}

\newcommand{\abs}{\left\lvert\,\strut#1\,\right\rvert} %Absolute value

\begin{document}

\section{Trigonométricas Elementales}
\begin{align}
\int\sen x\,\di x & = -\cos x + C \\
\int\cos x\,\di x & = \sen x + C \\
\begin{split}
\int\tan x\,\di x   & = -\ln\abs{\cos x} + C \\
& = \ln \abs{\sec x} + C
\end{split} \\
\int\csc x\,\di x & = -\ln\abs{\csc x- \cot x} + C \\
\int\sec x\,\di x & = -\ln\abs{\sec x+ \tan x} + C \\
\begin{split}
\int\cot x\,\di x & = \ln\abs{\cos x} + C \\
& = -\ln\abs{\csc x} + C \\
\end{split} \\
\int\sen^2 x\,\di x & = \frac x2-\frac 14\sen 2x + C \\
\int\cos^2 x\,\di x & = \frac x2+\frac 14\sen 2x + C \\
\int\tan^2 x\,\di x & =\tan x - x + C
\end{align}
\end{document}

• I suggest \newcommand{\di}{\mathop{}\!\mathrm{d}} that frees you from typing \, before it and works more generally. Also, besides having been deprecated for 20+ years, \rm does nothing relevant in the definition of \sen. – egreg Sep 22 '17 at 8:06
• Thank you @egreg. I will use your suggestion. As you pointed out, this can make me save some time and work. I did not know about \rm, I will take it into consideration in the future. – Cragfelt Sep 23 '17 at 9:23

"Basically the same" is not the same as "the same"... \documentclass{article}

\usepackage{amsmath}

\DeclareMathOperator{\di}{d\!}
\newcommand{\sen}{\mathop{\mathrm{sen}}\nolimits}
\newcommand{\abs}{\left\lvert\,\strut#1\,\right\rvert} %Absolute value

\begin{document}

\section{Trigonométricas Elementales}
\begin{align}
\int\sen x\,\di x & = -\cos x + C \\
\int\cos x\,\di x & = \sen x + C \\
\begin{split}
\int\tan x\,\di x   & = -\ln\abs{\cos x} + C \\
& = \ln \abs{\sec x} + C
\end{split} \\
\int\csc x\,\di x & = -\ln\abs{\csc x- \cot x} + C \\
\int\sec x\,\di x & = -\ln\abs{\sec x+ \tan x} + C \\
\begin{split}
\int\cot x\,\di x & = \ln\abs{\cos x} + C \\
& = -\ln\abs{\csc x} + C %\\ <---
\end{split} \\
\int\sen^2 x\,\di x & = \frac x2-\frac 14\sen 2x + C \\
\int\cos^2 x\,\di x & = \frac x2+\frac 14\sen 2x + C \\
\int\tan^2 x\,\di x & = \tan x - x + C
\end{align}

\end{document}

• I knew it! It has a ridiculously simple solution! I spent more time writting the question than you answering it! Thank you. – Cragfelt Sep 22 '17 at 5:46
• Optional but worthwhile: Provide a better \abs macro than the one provided by the OP. E.g., load the mathtools package and write \DeclarePairedDelimiter{\abs}{\lvert}{\rvert}. – Mico Sep 22 '17 at 5:51
• Excellent @Mico!. I will take that into consideration. Thank you for the advice. – Cragfelt Sep 22 '17 at 6:02
• @Mico I changed the command declaration as you suggested, but the new command does not work for "tall" equations like fractions or big operators with \displaystyle. – Cragfelt Sep 23 '17 at 9:26
• @Cragfelt - Please read the manual of the mathtools package for how \abs macro works. If you want auto-sized "fences", you need to type \abs*{...}. – Mico Sep 23 '17 at 10:10