
\subsection{\bf Formula for Stiffness Matrix Element---II}

\begin{slide}
\begin{itemize}

\vspace{5pt}
\begin{center}
\centerline{\psfig{figure=cps615.fig6.ps,clip=true,width=4.0in}}
Figure: FEM 6
\end{center}
\vspace{5pt}

\item If $i$ and $j$ are not connected in the triangulation, then
$S_{ij} = 0$.  If they are connected, written $i\leftrightarrow j$
and $i\neq j$ then $S_{ij} = - \left( \cot \theta_1 + \cot \theta_2
\right)$ (Figure~FEM~5)

\item Example:\ \ Show that
\[
\sum_i S_{ii} + \sum_{i\neq j} S_{ij} = 0
\]

Hint: $\varphi (\underline{x}) = 1$ is a solution to Laplace's
equation for which approximations are exact
\end{itemize}

\end{slide}

