A sphere is inscribed in the tetrahedron whose vertices are $$A = (6,0,0), B = (0,4,0), C = (0,0,2),$$ and $$D = (0,0,0).$$ The radius of the sphere is $$m/n,$$ where $$m$$ and $$n$$ are relatively prime positive integers. Find $$m + n.$$

(第十九届AIME1 2001 第12题)