# : “Display math should end with $$” error? I'm pretty new to LaTeX, completely new actually, and there is one thing I can't quite understand. I get the error "Display math should end with$$" when writing an equation like this:

$$R_{2}=\frac{U_{R_2}}{I_2}=\frac{2,95\, \mathrm{V}}{0,03*10^{-3}\, \mathrm{A}} = 7,10\,\mathrm{k}\Omega \,. \label{eq:Bsp_OhmsLaw}$$


I tried a lot of things, I also skimmed these forums. Nothing seemed to help :-(

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Welcome to the site! You shouldn't use all of those $ symbols within the equation environment; remove them, and the blank lines, and you should get closer to a working piece of code :) – cmhughes Mar 29 '14 at 15:57 Please post complete compilable code as this makes it much easier to understand and answer your question. – cfr Mar 29 '14 at 16:00 Also,$$is from plain TeX and has been replaced in LaTeX with a host of options, including $and$. – John Kormylo Mar 29 '14 at 16:04 ## 2 Answers In addition to removing the $ as cmhughes suggested, the blank lines in the equation environment confuse things. This may be why you added the dollar signs - with the blanks, LaTeX otherwise complains about missing $ symbols in the code. Here's a working version: \documentclass{article} \begin{document} $$R_{2}=\frac{U_{R_2}}{I_2}=\frac{2,95\, \mathrm{V}}{0,03*10^{-3}\, \mathrm{A}} = 7,10\,\mathrm{k}\Omega \,. \label{eq:Bsp_OhmsLaw}$$ \end{document}  - Thank's so much! It may sound odd, but my professor never really expained why and how to properly use the$ signs, so I just use them before and after every term that isn't just text.. worked fine so far, but I can see it leads to the first problems already. –  user48884 Mar 29 '14 at 16:20
Bascially, equation already tells TeX it is in maths mode. That's why you need e.g. \mathrm{} to get something text-like. If you are in a maths environment like that, don't use $ signs. Normally, you'd use $ signs if you wanted to typeset some maths inline i.e. in the middle of a paragraph or in the cell of a table or something like that. –  cfr Mar 29 '14 at 16:43
@cfr Or, even better than $...$, you can use $$...$$. –  Jubobs Apr 11 '14 at 12:25
You could also change * to \times to get a properly typeset multiplication symbol, and even though it is obvious from context in this case, * is used for other operations in mathematics. –  Ahlqvist Apr 11 '14 at 12:59
I'd also like to mention that it's generally a good idea to not use several equals signs in the same line. –  1010011010 May 16 '14 at 22:32

Use the siunitx package for typesetting physical quantities:

\documentclass{article}

\usepackage{amsmath}
\usepackage[
locale = DE % comma as decimal mark
]{siunitx}

\begin{document}

$$\label{eq:Bsp_OhmsLaw} R_{2} = \frac{U_{2}}{I_{2}} = \frac{\SI{2.95}{\volt}}{\SI{0.03e-3}{\ampere}} = \SI{7.10}{\kilo\ohm}.$$
Ohm's law is used in equation~\eqref{eq:Bsp_OhmsLaw}.

\end{document}


Note that cfr has given the answer to why the initial code isn't compiling.

Update

You can make the code a bit shorter by use abbreviations for the physical units:

\documentclass{article}

\usepackage{amsmath}
\usepackage[
locale = DE % comma as decimal mark
]{siunitx}

\begin{document}

$$\label{eq:Bsp_OhmsLaw} R_{2} = \frac{U_{2}}{I_{2}} = \frac{\SI{2.95}{\V}}{\SI{0.03e-3}{\A}} = \SI{7.10}{\kohm}.$$
Ohm's law is used in equation~\eqref{eq:Bsp_OhmsLaw}.

\end{document}


(See page 37 of the manual.)

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Wouldn't it be more instructive to write out \V and \A as it was done with \kiko\ohm. Also unless really really required to, I'd recommend using . as the decimal point, not comma. Otherwise one will have to wrap all non unit numbers in \num{...} which is a big hassle if one have not done so from the beginning. –  daleif Apr 11 '14 at 10:55
@daleif Regarding \V and \A: I guess. (The reverse case: Do you know if there is an abbreviation for \kiko\ohm?) The comma is because that is how it is in the question. –  Svend Tveskæg Apr 11 '14 at 12:21
abreviations \kohm –  daleif Apr 11 '14 at 12:24
@daleif You could also use the icomma package to get properly spacing if comma is used a decimal separator. –  Ahlqvist Apr 11 '14 at 13:01
In that case I'd rather use the siunitx approach and not have globally active math comma –  daleif Apr 11 '14 at 13:18