I only installed LaTeX just over 3 weeks ago and I've written about 20 pages of Maths coursework. I have a bit of an issue with arrays - I am trying to use something like dcases to show a piecewise function. My function involves fractions and they come out too small but I can't seem to follow other people's answers to this problem because I am not familiar with the packages etc. Please could you look at my document set-up, as below, and suggest what I'm doing wrong? When I use the amsmath package, things disappear off the ends of pages and everything moves!

\documentclass[11pt]{article}       % use "amsart" instead of "article" for AMSLaTeX format
%\usepackage{geometry}                      % See geometry.pdf to learn the layout options. There are lots.
%\geometry{a4paper}                         % ... or a4paper or a5paper or ... 
%\geometry{landscape}                       % Activate for for rotated page geometry
%\usepackage[parfill]{parskip}          % Activate to begin paragraphs with an empty line rather than an indent
%\usepackage{graphicx}              % Use pdf, png, jpg, or eps with pdflatex; use eps in DVI mode
                                % TeX will automatically convert eps --> pdf in pdflatex        
\usepackage[margin=0.8in,include foot]{geometry}
\lhead{Chloe Jones A2460906}
\chead{TMA MST326 03}
\title{TMA MST326 03}
%\author{Chloe Jones A2460906}
%\date{March 2015}                          % Activate to display a given date or no date

H= \begin{array}{lr}
0 & : \textrm{n even,}\ \  n\neq2\\
 \frac{5\pi + 2n\pi +  5\pi - 2n\pi  }{ \pi ^2 \left({4 - n^2  }\right)}    & :  \textrm{n odd}

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    Can you make your code into a minimal working example (MWE)? There is nothing wrong with what you've posted. We need to see a working example in order to help you resolve the issue. – cfr Mar 29 '15 at 15:12
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    Please, may you give us a image with the formula that you would like write. – Giacomo Alessandroni Mar 29 '15 at 15:23
  • Sorry I have limited time for a coursework deadline - I've changed my values here - and I would find it too difficult to post an image but I would be grateful if you could help with my code please. I was just trying to input a large brace for a piecewise function. The function is zero for odd n and the value of the function is a fraction involving n when n is odd. Thank you for your replies. – Chloe Mar 30 '15 at 10:15

The cases environment, provided by the amsmath package, is ready-made for your math expression. Note that the cases environment renders each line in text-style math mode (aka inline math mode). If you don't need to conserve space, consider using a dcases environment instead: It displays the fractional term in display-style math mode and also increases the space between rows.

Either way, the entire expression, starting with H=, has to be in math mode. This can be achieved by placing it in a \[ ... \] group.

enter image description here

\usepackage{mathtools} % loads 'amsmath' package, provides 'dcases' env.
Using a \texttt{cases} environment:
H= \begin{cases}
0 & \text{if $n$ even, $n\neq2$}\\
\frac{5\pi + 2n\pi +  5\pi - 2n\pi}{\pi^2 (4 - n^2)} & \text{if $n$ odd}

Using a \texttt{dcases} environment:
H= \begin{dcases}
0 & \text{if $n$ even, $n\neq2$}\\
\frac{5\pi + 2n\pi +  5\pi - 2n\pi}{\pi^2 (4 - n^2)} & \text{if $n$ odd}

Addendum -- As @daleif has observed in a comment, the dcases* environment may also be of interest. It differs from the dcases environment in that the material after the & character is automatically placed in text mode, obviating the need for the \text{...} wrappers. Using a dcases* environment, one could write the solution as follows:

H= \begin{dcases*}
0 & if $n$ even, $n\neq2$\\
\frac{5\pi + 2n\pi +  5\pi - 2n\pi}{\pi^2 (4 - n^2)} & if $n$ odd
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    Even funnier: dcases* then you don't need \text in the second column (hardly any is using that version) – daleif Dec 20 '15 at 11:26
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    @daleif - Thanks for this note. I've provided an addendum to illustrate its use for the example at hand. – Mico Dec 20 '15 at 20:18
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    Your wording is a little off, shouldn't it be "after the & char is..." – daleif Dec 20 '15 at 20:19
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    But shouldn't it still say "after the & character" , that $ char does not make sense? – daleif Dec 20 '15 at 20:31
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    @daleif - Two typos, back to back. :-( – Mico Dec 20 '15 at 20:34

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