The image you provided seems to be a geometry problem written in Vietnamese. Here's a translation of the problem:

Question 4 (4.0 points)

Given a circle with center $O$ and radius $AD$. The chords $AC$ and $BD$ intersect at point $E$, where $E$ lies inside the circle. The line $EF$ is perpendicular to $AD$ at $F$. Prove the following:

a) The quadrilateral $ABEF$ is cyclic.
b) The line $FE$ is the angle bisector of $\angle BFC$.
c) Point $E$ is the incenter of triangle $BCF$.

Let's break this down and start solving each part one by one. Would you like me to go through each part of the proof in detail?