We will consider two angles as follows:
[tex]<\text{ CFD > \& < DFE >}[/tex]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.
[tex]CF\text{ , FD }[/tex][tex]&[/tex][tex]DF,\text{ FE}[/tex]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:
[tex]FD\text{ = DF}[/tex]Hence, the ray common to both < CFD > and < DFE > is :
[tex]\textcolor{#FF7968}{FD}[/tex]