Then, since the point F is the centre of the circle ABC, FA is equal to FC.
Again, since the point G is the centre of the circle ADE, GA is equal to GD.
But FA was also proved equal to FC;
therefore FA, AG are equal to FC, GD,
so that the whole FG is greater than FA, AG;
but it is also less [I. 20]: which is impossible.
Therefore the straight line joined from F to G will not fail to pass through the point of contact at A;
therefore it will pass through it.
而己庚線、截兩圜界於戊、於丙。令於切界作乙己、乙庚、兩線。其乙己庚角形之己乙、乙庚、兩邊幷。大
於己庚一邊。而乙庚與庚戊。乙己、與己丙。俱同心所出線。宜各等。卽庚戊、丙己、兩線幷。亦大於庚己一線矣。一卷二十
夫庚己線。分為庚戊、丙己。尚餘丙戊。
而云庚戊、丙己。大於庚己。則分大於全也。
故直線聯己庚。
必過乙。