Completing the Square: Circle

The radius $r$  of a circle inscribed within three mutually externally tangent circles of radii $a$, $b$, and $c$ is given by

$\frac{1}{r} = \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + 2 \sqrt{\frac{1}{ab} + \frac{1}{ac} + \frac{1}{bc}}.$
What is the value of  $r$ when $a=4$, $b=9$ and $c=36$?