79. In a triangle, three lines parallel to its sides and tangent to the inscribed circle have been drawn. They cut off three triangles from the given one. The radii of the circumcircles of these triangles are $R_{1}, R_{2}, R_{3}$. Find the radius of the circumcircle of the given triangle.
79. Prove that if $a, b, c$ are the lengths of the sides of a triangle, then the perimeters of the cut-off triangles will be $2(p-a), 2(p-b)$, $2(p-c)$. Consequently, if $R$ is the radius of the circumscribed circle, then $R_{1}+R_{2}+R_{3}=\left(\frac{p-a}{p}+\frac{p-b}{p}+\frac{p-c}{p}\right) R=R$.

Answer: $R=R_{1}+R_{2}+R_{3}$.