+13 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 \(c=36\)?
