Your code isn't compilable since it contains macros such as \textgreater
and \textless
that shouldn't occur in math mode. The argument of a \label
command has to be enclosed in curly braces, by the way.
You didn't give any hints as to your criteria for beauty and legibility, so I'll have to use my own. :-) There's nothing terribly wrong with the look once the code's been fixed so that it compiles. (I'm talking purely about the aestetics, of course; I can't tell if the math is correct or not...) I did notice that the overall look is quite cramped. You could use a dcases*
environment (provided by the mathtools
package) instead of the basic cases
environment to provide a bit more whitespace. Next, there's a minor issue with the uneven usage of accents: the "hats" are uniformly "wide", but the "bars" are not. I suggest using \overline
for the Cl
terms. Third, I'd get rid of two redundant pairs of parentheses on each line. Finally, since there's no point in providing a \label
to an unnumbered equation, I'd either comment it out or get rid of it entirely.

\documentclass[12pt, a4paper]{book}
\usepackage{mathtools} % for 'dcases*' environment
\begin{document}
Before:
\begin{equation*}
\theta({P,\bar{Cl}}) =
\begin{cases}
S(P,\widehat{Cl}) + (S(P,\widehat{Cl}) \times ((\bar{R}-\widehat{R})/\bar{R})) & \widehat{Cl} < \bar{Cl} \\
S(P,\widehat{Cl}) - (S(P,\widehat{Cl}) \times ((\widehat{R}-\bar{R})/\bar{R})) & \widehat{Cl} > \bar{Cl}
\end{cases}
\label{eq: state}
\end{equation*}
After:
\begin{equation*}
\theta(P,\overline{Cl}\,) =
\begin{dcases*}
S(P,\widehat{Cl}) + S(P,\widehat{Cl}) \times (\bar{R}-\widehat{R})/\bar{R}
& if $\widehat{Cl} < \overline{Cl}$ \\
S(P,\widehat{Cl}) - S(P,\widehat{Cl}) \times (\widehat{R}-\bar{R})/\bar{R}
& if $\widehat{Cl} > \overline{Cl} $
\end{dcases*}
\end{equation*}
\end{document}
\frac
what is the difference betweenfrac
andfrax
or is it a typo? – tandem Oct 7 '14 at 13:26