Then, since AF is equal to FB, and FE is common, 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.
Therefore CD, which is through the centre, and bisects AB which is not through the centre, also cuts it at right angles.
卽甲乙己角形之乙己。與甲丁己角形之丁己。兩邊等。甲己同邊。
甲乙、甲丁、兩線、俱從心至界。又等。卽兩形等。
則其對等邊之甲己乙、甲己丁。亦等。一卷八
而為兩直角矣。