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 = 90o- 1/2*arc ED.
Angle EFC = 1/2*arc EC = 1/2*(arc CD - arc ED) = 90o- 1/2*arc ED.
=> Angle A = Angle EFC.
Angle A + Angle EFB = Angle EFC + Angle EFB = 180o.
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,