If  you want to typeset (advanced) mathematics, you should use AMS-LATEX.  The AMS-LATEX bundle is a collection of packages and classes for mathematical typesetting.  AMS-LATEX is produced by The American Mathematical Society and it is used extensively for mathematical typesetting.  LATEX itself does provide some basic features and environments for mathematics, but they are limited and in some cases inconsistent.  

AMS-LATEX is a part of the required distribution and is provided with all recent LATEX distributions.  We assume amsmath is loaded in the preamble; \usepackage{amsmath}.