H
E
E
L
E
M
E
N
T
S
since CF is parallel to AD, and BD has fallen on them,
the exterior angle CGB is equal to the interior and oppostie angle ADB. [I.29]
But
the angle ADB is equal to the angle ABD,
since
the side BA is also equal to AD; [I.5]
therefore
the angle CGB is also equal to the angle GBC,
so that
the side BC is also equal to the side CG. [I.6]
But
CB is equal to GK, and CG to KB; [I.34]
therefore GK is also equal to KB; therefore
CGKB is equilateral.
