H
E
E
L
E
M
E
N
T
S
let another straight line be inflected, and let there be another angle BDC;
let
DE be joined and produced to G.
Similarly then
we can prove that the angle GEC is double of the angle EDC,
of which
the angle GEB is double of the angle EDB;
therefore
the angle BEC which remains is double of the angle BDC.
