\documentclass[a4paper,12pt]{article}
\usepackage{epsfig}
\newcommand{\un}{\underline}
\begin{document}
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\begin{center}
\textbf{Vector Fields}

\textit{\textbf{Scalar and Vector Fields}}
\end{center}

\textbf{Question}

Sketch the following plane vector field and determine its field
lines.

$\un{F}(x,y) = \cos \un{i} -\cos x \un{j}$


\textbf{Answer}

$\begin{array}{l}
\textrm{The field lines satisfy }\frac{dx}{\cos y} =-\frac{dy}{\cos x},\\
\textrm{that is }\cos x dx +\cos y dy = 0.\\
\textrm{Thus they are the curves }\\
\sin x +\sin y =C.
\end{array}
\ \ \
\begin{array}{c}
\epsfig{file=VF-1A-8.eps, width=40mm}
\end{array}$

\end{document}