Extra

Er geldt: $
\Delta DQB \sim \Delta RQC
$

In driehoek PRZ geldt: DC = 8 en PZ = 4.

$
\begin{array}{l}
PR^2 = 4^2 + 8^2 = 16 + 64 = 80 \\
PR = \sqrt {80} = 4\sqrt 5 \\
\end{array}
$

Conclusie

$
PR = 4\sqrt 5
$