As we all know, there are many equation environments to choose, including
$$for inline equation
equationfor single-line equation
IEEEeqnarrayfor multi-line equation and equations
To use IEEEeqnarray, you should install IEEEtran, which can be downloaded at http://mirror.ctan.org/macros/latex/contrib/IEEEtran.zip
On Arch Linux, using texlive, I just use texconfig -> CONF to find the path of TEXMFLOCAL, and make a folder TEXMFLOCAL/tex/latex/IEEE and copy all the files in that folder. Finally, run texhash.
You should add this line in the preamble of your LaTeX file
To add an IEEEeqnarray, use