Hello everyone,

I'm having a bit of trouble understanding the following proof of this problem:

"Prove that ABFE is a chordal quadrileteral."

Proof given by solutions book:

"Angle A = 1/2*arc CD - 1/2*arc ED = 90

^{o}- 1/2*arc ED.

Angle EFC = 1/2*arc EC = 1/2*(arc CD - arc ED) = 90

^{o}- 1/2*arc ED.

=> Angle A = Angle EFC.

Angle A + Angle EFB = Angle EFC + Angle EFB = 180

^{o}.

Thus ABFE is a chordal quadrileteral."

I do understand the logic of the proof and the whole conclusion, however I am stuck at the beginning, where the angles are given as arcs. Could anybody explain to me how this is done?

Thanks in advance,

Tom