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IMO 1970 — Official contest Question 1

IMO · IMO 1970 — Official contest

Question 1

Given a point $M$ on the side $AB$ of the triangle $ABC$, let $r_1$ and $r_2$ be the radii of the inscribed circles of the triangles $ACM$ and $BCM$ respectively while $\rho_1$ and $\rho_2$ are the radii of the excircles of the triangles $ACM$ and $BCM$ at the sides $AM$ and $BM$ respectively. Let $r$ and $\rho$ denote the respective radii of the inscribed circle and the excircle at the side $AB$ of the triangle $ABC$. Prove that
$$\frac{r_1r_2}{\rho_1\rho_2}=\frac r\rho.$$