If two circles touch one another internally, and their centers are found, then the straight line joining their centers, being produced, will fall upon the point of union of the circles. * For let two circles, $ABC$ and $ADE$, touch one another internally at point $A$, and let the center $F$ of circle $ABC$ have been found [Prop. 3.1], and (the center) $G$ of (circle) $ADE$ [Prop. 3.1]. * I say that the straight line joining $G$ to $F$, being produced, will fall on $A$.
If two circles touch one another internally at a point $A$, then the straight line going through their centers goes also through $A$.
Proofs: 1