1996 OIM Problems/Problem 2

Problem

Let $M$ be the midpoint of the median $AD$ of triangle $ABC$ ($D$ belongs to side $BC$). The line $BM$ cuts the side $AC$ at the point $N$. Prove that $AB$ is tangent to the circumcircle of the triangle $NBC$ if, and only if, the following equality is verified

\[\frac{\overline{BM}}{\overline{MN}} = \frac{(\overline{BC})^2}{(\overline{BN})^2}\]

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also

https://www.oma.org.ar/enunciados/ibe11.htm