Question 133
As shown in the figure, $BC$ and $AC$ are tangent to the inscribed circle $I$ of $\triangle ABC$ at $M$ and $N$. $E$ and $F$ are the midpoints of $AB$, $AC$ respectively. $EF$ intersects $BI$ at $D$. Prove that $M$, $N$, $D$ are collinear.
