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I have trying to use new command to represent math, and this is what I type in:

\\
\newcommand\fpi2[2][\pi][2]{frac{#1}{#2}}\\
\newcommand\mylim[2][x]{lim_{#1\to #2}}

I'm trying to create a new command which represent $frac{\pi}{2}$ Is it possible for me to use \mylim{${\fpi2}^-$}?

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Unless you use a \csname ... \endcsname workaround method, you can't have a non-letter (such as the numeral 2) be part of a command name. Use a command name such as \fpi or \fpitwo instead. –  Mico Sep 11 '13 at 16:58
    
You have posted several questions in the last few days repeating the same error every time: you need a backslash \frac and \lim not frac and lim (in addition to the point of this question that you can not have a command \fpi2.) –  David Carlisle Sep 11 '13 at 18:30
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marked as duplicate by Werner, egreg, Heiko Oberdiek, lockstep, Stefan Kottwitz Sep 11 '13 at 17:45

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1 Answer

The name of a command should contain only letters (not absolutely true, but at the moment let's accept it). Apparently your command doesn't require arguments, since you are interested in a constants string:

\documentclass{article}

\newcommand\fpi{\frac{\pi}{2}}
\newcommand\mylim[2][x]{\lim_{#1\to #2}}

\begin{document}

$\fpi\quad\mylim{\fpi^-}$

\[
\fpi\quad\mylim{\fpi^-}
\]

\end{document}

enter image description here

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Thank you so much :D –  Huang Elizabeth Sep 11 '13 at 17:00
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@HuangElizabeth You're welcome! Don't forget that (after a sensible wait) you can accept answers by clicking the checkmark to their left :-) –  Gonzalo Medina Sep 11 '13 at 17:03
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