# Circles and tangents

3 circles of radius 3 cm, 4cm, 5 cm touch each other externally at $A$, $B$, $C$. Tangents drawn at $A$, $B$, $C$ intersect at $P$.

Find $PA + PB + PC$ .

Thanks.

My thoughts and approach:

• First hint: what can you say on the length of $PA$ compared to the length of $PB$? – TZakrevskiy Jul 5 '14 at 13:51
$PA+PB+PC$ is just three times the inradius of a triangle having side lengths $(a,b,c)=(7,8,9)$. Since $$r(a+b+c) = 2\Delta,$$ we only need to find the area of the triangle thorugh Heron's formula. We have: $$\Delta^2 = p(p-a)(p-b)(p-c) = 12\cdot 3\cdot 4\cdot 5,$$ hence $\Delta=12\sqrt{5}$ and: $$PA+PB+PC = 3r=\frac{6\Delta}{a+b+c} = 3\sqrt{5}.$$