H
E
E
L
E
M
E
N
T
S
two sides are equal to two sides; and
the base EA is equal to the base EB;
therefore
the angle AFE is equal to the angle BFE. [I.8]
But, when a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right; [I.Def.10]
therefore
each of the angles AFE, BFE is right.
