We will consider two angles as follows:

We are to determine the rays that have a common side to both < CFD > and < DFE >.
We will go ahead and make combination of pairs for each angle < F > common to both.



The common sides of the angle rays would contain similar letters! From the two combinations for each ray-angles we can see that the side:

Hence, the ray common to both < CFD > and < DFE > is :
