MathNet 2014 Question 5

Let $ABC$ be a triangle with $\angle A=40^\circ$, $\angle B=60^\circ$. Choose $D\in AC$ and $E\in AB$ such that $\angle CBD=40^\circ$ and $\angle ECB=70^\circ$. Let $F=BD\cap CE$. Prove $AF\perp BC$.