MathNet 2021 — Part 01 Question 88

In $\triangle ABC$, suppose the incircle has center $I$ and is tangent to $BC$ at $D$, and the $A$ excircle has center $I_{a}$ and is tangent to $BC$ at $D'$. Show that $ID'$ and $I_{a}D$ intersect on the altitude from $A$ to $BC$.