\documentclass[a4paper,12pt]{article}
\usepackage{epsfig}
\newcommand{\un}{\underline}
\begin{document}
\parindent=0pt


\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.

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


\textbf{Answer}

$\begin{array}{l}
\textrm{The field lines satisfy }\frac{dx}{x} =\frac{dy}{y}\\
\rm{Thus\ }\ln y =\ln x + \ln C, \ \rm{or} \ y=Cx.\\
\textrm{The field lines are straight half-lines}\\
\textrm{emanating from the origin}
\end{array}
\ \ \ \ \
\begin{array}{c}
\epsfig{file=VF-1A-2.eps, width=40mm}
\end{array}$


\end{document}