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

\begin{center}
\textbf{Applications of Partial Differentiation}

\textit{\textbf{Extremes}}
\end{center}

\textbf{Question}

Find and classify the critical points of the function

$$f(x,y)=xy-x+y$$


\textbf{Answer}

\begin{eqnarray*}
f_1 & = & y-1\\
f_2 & = & x+1\\
A & = & f_{11} =0\\
B & = & f_{12} =1\\
C & = & f_{22} =0
\end{eqnarray*}
Critical point $(-1,1)$ is a saddle point since $B^2-AC>0$.

\end{document}