equationsmacrosspacing
The following code produces a negative phantom (from the comment in this answer):
\newcommand{\negphantom}[1]{\settowidth{\dimen0}{#1}\hspace*{-\dimen0}}
However, when used inside an equation like
$$a\negphantom{\inf_{t>0}t^2}b$$
I get an error. The solution is to do
$$a\negphantom{$\inf_{t>0}t^2$}b$$
Is it possible to define negphantom in such a way that it respects being inside an equation or not? (the latter solution is not perfect, since the \inf is not in displaystyle).
without using amsmath:
\documentclass{article}
\begingroup
\catcode`(\active \xdef({\left\string(} % ( is defined as \left(
\catcode`)\active \xdef){\right\string)} % ) is defined as \right)
\endgroup
\mathcode`(="8000 \mathcode`)="8000 % active in mathmode
\begin{document}
\[
(\frac{(3-\frac{1}{x})^2}{(\frac4x-1)x})
\]
\end{document}
and if you are using package amsmath
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\def\resetMathstrut@{%
\setbox\z@\hbox{%
\mathchardef\@tempa\mathcode`\[\relax
\def\@tempb##1"##2##3{\the\textfont"##3\char"}%
\expandafter\@tempb\meaning\@tempa \relax
}%
\ht\Mathstrutbox@\ht\z@ \dp\Mathstrutbox@\dp\z@}
\makeatother
\begingroup
\catcode`(\active \xdef({\left\string(}
\catcode`)\active \xdef){\right\string)}
\endgroup
\mathcode`(="8000 \mathcode`)="8000
\begin{document}
\begin{align}
f(x) &= (\frac{(3-\frac{1}{x})^2}{(\frac4x-1)x}-1)x^2
\end{align}
\end{document}
You can do with mathtools, but I wouldn't recommend it. This leaves a gap on the right of “sin”, but centering it in the available space would be even worse.
\documentclass{article}
\usepackage{mathtools}
\let\sin\relax % remove the previous definition of \sin
\DeclareMathOperator{\sin}{%
\mathrlap{\operatorname{sin}}\hphantom{\cos}%
}
\begin{document}
$\sin\alpha$
$\cos\alpha$
$e^{\sin x}$
$e^{\cos x}$
\end{document}
A different solution, where you add the space after the \sin<expression> can be
\documentclass{article}
\usepackage{mathtools}
\newcommand{\csin}[1]{%
\mathop{}\!\mathrlap{\sin#1}\hphantom{\cos#1}%
}
\begin{document}
$3\csin{\alpha}+\cos\beta$
$3\cos\alpha+\cos\beta$
$e^{\csin{x}+i\cos{x}}$
$e^{\cos x+i\cos x}$
\end{document}
Since we need to know what the argument to \csin is, braces delimiting it are necessary.
However, this has limitations because it's difficult to catch the type of the last object in the argument of \csin, so this might break with parenthesized expressions and hand corrections would be needed, for instance
\csin{(x+y)}\mathclose{}
Yet another proposal: a \sincorr space to be added by hand where necessary:
\documentclass{article}
\newcommand{\sincorr}{\mathpalette\dosincorr\relax}
\newcommand{\dosincorr}[2]{%
\sbox0{$#1\cos$}\sbox2{$#1\sin$}%
\kern\dimexpr\wd0-\wd2\relax}
\begin{document}
$3\sin\alpha\sincorr+\cos\beta$
$3\cos\alpha+\cos\beta$
$e^{\sin x\sincorr+i\cos x}$
$e^{\cos x+i\cos x}$
\end{document}
The output is identical as with the previous solution.
Best Answer
You check for math mode and then use
\mathpalette:Now that we're sure the space is correct in all cases, we can remove the debugging bits and write down the final definition: