[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&S&(~ \frac{1}{2} &,& 1~)
% (c,d)
&T&(~ \frac{1}{2} &,& 4~)
\end{array}
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{4-1}{\frac{1}{2}-\frac{1}{2}}\implies \stackrel{und efined}{\cfrac{3}{0}}[/tex]
two points with an undefined slope, means a vertical line, check the picture below.
