Question 99
Let $O_{1}$ be a point in the exterior of the circle $c(O, R)$ and let $O_{1}N$, $O_{1}D$ be the tangent segments from $O_{1}$ to the circle. On the segment $O_{1}N$ consider the point $B$ such that $BN = R$. Let the line from $B$ parallel to $ON$ intersect the segment $O_{1}D$ at $C$. If $A$ is a point on the segment $O_{1}D$ other than $C$ so that $BC = BA = a$, and if $c^{\prime}(K, r)$ is the incircle of the triangle $O_{1}AB$; find the area of $ABC$ in terms of $a$, $R$, $r$.