“Just for fun with” TikZ …
Code (three arbitrary coordinates)
\documentclass[tikz]{standalone}
\newcommand*{\triangleBaseWidth}{2cm}
\newcommand*{\triangleHeight}{2cm}
\newcommand*{\triangleCountOfLines}{5}% for non-animation
\usetikzlibrary{calc}% for ($()!!()$) calculation
\begin{document}
\foreach \triangleCountOfLines in {1,...,10}{% animation
\begin{tikzpicture}[font=\tiny]
\path (0,0) coordinate (A)
+ (\triangleBaseWidth,0) coordinate (B)
+ (\triangleBaseWidth/2,\triangleHeight) coordinate (C);% the triangle's points
\draw (A) --
node[below] {$n = \triangleCountOfLines$}% n = ?
(B) -- (C) -- cycle;% the triangle
\foreach \l in {1,...,\triangleCountOfLines}{% horizontal lines
\draw ($(C)!\l/(\triangleCountOfLines+1)!(A)$) -- ($(C)!\l/(\triangleCountOfLines+1)!(B)$);
}
\foreach \c/\p in {A/left,B/right,C/above}{% labels of coordinates
\fill (\c) circle (.8pt) node[\p] {\c};
}
\end{tikzpicture}
}% animation
\end{document}
Output

Code (isosceles triangle shape)
\documentclass[tikz]{standalone}
\newcommand*{\triangleBaseWidth}{2cm}
\newcommand*{\triangleHeight}{2cm}
\newcommand*{\triangleCountOfLines}{5} % for non-animation
\usetikzlibrary{calc} % for ($()!!()$) calculation
\usetikzlibrary{shapes.geometric} % for the isosceles triangle
\begin{document}
\foreach \triangleCountOfLines in {1,...,10}{% animation
\begin{tikzpicture}[font=\tiny]
\node[
isosceles triangle,
anchor=center,
draw,
rotate=90,
minimum width=\triangleBaseWidth,
minimum height=\triangleHeight,
inner sep=0pt,
] (3) at (0,0) {};
\node[below] at (3.lower side) {$n = \triangleCountOfLines$};% n = ?
\begin{scope}
\clip (3.left corner) -- (3.right corner) -- (3.apex) -- cycle;
\foreach \l in {1,...,\triangleCountOfLines}{% horizontal lines
\draw ($(3.apex)!\l/(\triangleCountOfLines+1)!(3.left corner)$) -- ($(3.apex)!\l/(\triangleCountOfLines+1)!(3.right corner)$);
}
\end{scope}
\end{tikzpicture}
}% animation
\end{document}
Output

TeXtoEPS. it is really outdated – Sep 05 '12 at 14:19