Does not compile with complete text but, when I compile it separately, it gives me the required output. What should I do?

Question

\begin{eqnarray*}
M&=& \max{\left \{a,b,\frac{a+b}{2},\right}\\
&& a+b,a+b/2\Bigg\}
\end{eqnarray*}

Answer 1

I recommend the use of package amsmath, some options with environment split:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{gather*}
  \begin{split}
    M = {} & \max\biggl\{ a, b, \frac{a+b}{2},\\
           & a + b, a + \frac{b}{2} \biggr\}
  \end{split}
\end{gather*}

\begin{gather*}
  \begin{split}
    M = \max\biggl\{& a, b, \frac{a+b}{2},\\
                    & a + b, a + \frac{b}{2} \biggr\}
  \end{split}
\end{gather*}

\begin{gather*}
  \begin{split}
    M = {} & \max\{ a, b, (a+b)/2,\\
           & a + b, a + b/2\}
  \end{split}
\end{gather*}
\end{document}

Answer 2

To answer the question, you get

! Missing delimiter (. inserted).

Why do you get “correct” output? Because TeX adds . after \right, which is missing a delimiter after it; then the } balances the { after \max and all gets in sync. You might fix it by adding the period yourself and removing the useless braces

\begin{eqnarray*}
M&=& \max\left \{a,b,\frac{a+b}{2},\right.\\
&& a+b,a+b/2\Bigg\}
\end{eqnarray*}

You get

but this is wrong to begin with.

Firstly, because eqnarray is buggy and should not be used for serious work: use amsmath environments instead (see eqnarray vs align). Secondly, it's apparent that the braces have different size.

When you split delimiters across lines, both should be given the same size, either automatically or manually. In the case of standard two story fractions the size is \bigg; but they should be \biggl and \biggr (for left and right) in order to get correct horizontal spacing. Hence

\begin{eqnarray*}
M&=& \max\biggl\{a,b,\frac{a+b}{2},\\
&& a+b,a+b/2\biggr\}
\end{eqnarray*}

might do, but if you look closely, the spaces around the equals sign are awfully big:

Do you notice the differences with the top picture? Here is what we get with \Bigl and \Bigr instead:

Depending on personal preference, you may want to use such smaller size.

Anyway, you should follow Heiko's suggestions as explained in his answer.

\documentclass[twocolumn]{article}
\usepackage{amsmath}

\usepackage{lipsum} % just for giving context

\begin{document}

\lipsum*[1]
\begin{equation*}
\begin{split}
M=\max\biggl\{ & a,b,\frac{a+b}{2},\\
              & a+b,a+\frac{b}{2}
      \biggr\}
\end{split}
\end{equation*}
\lipsum[2-15]

\end{document}