\documentclass{minimal}

\usepackage{amsmath}

\begin{document}

\[
P_p = 
\begin{pmatrix}
\frac{2n}{r-l} & 0 & \frac{r+l}{r-l} & 0 \\
0 & \frac{2n}{t-b} & \frac{t+b}{t-b} & 0 \\
0 & 0 & -\frac{(f+n)}{f-n} & -\frac{2 f n}{f-n} \\
0 & 0 & -1 & 0
\end{pmatrix}
\]

\[
\varphi_{\textrm{fovy}} = 2 \cdot \tan^{-1}(\frac{1}{P_p(1,1)})
\]

\[
a = \frac{w}{h} = \frac{r-l}{t-b} = \frac{(r-l)2n}{(t-b)2n} = \frac{\frac{2n}{t-b}}{\frac{2n}{r-l}} = \frac{P_p(1,1)}{P_p(0,0)}
\]

\end{document}
