Fall Mathematical Competition

Let $M$ and $N$ be the midpoints of the sides $AC$ and $BC$ of $△ABC$ ($AC>BC$) and let the bisector of $\angle B$ intersect the segment $MN$ at a point $P$. The incircle of $△ABC$ has center $I$ and is tangent to $BC$ at a point $Q$. Denote by $R$ the intersection point of the perpendiculars from $P$ and $Q$ to $MN$ and $BC$, respectively, and by $S$ the intersection point of the lines $AB$ and $RN$.

a) Prove that the quadrilateral PCQI is cyclic. b) Express the length of the segment BS by the lengths a, b, c of the sides of $△ABC$.