Proposition 9.
If a point is taken within a circle, and more than two equal straight lines fall from the point on the circle, then the point taken is the center of the circle.
Proposition 10.
A circle does not cut a circle at more than two points.
Proposition 11.
If two circles touch one another internally, and their centers are taken, then the straight line joining their centers, being produced, falls on the point of contact of the circles.
Proposition 12.
If two circles touch one another externally, then the straight line joining their centers passes through the point of contact.
Proposition 13.
A circle does not touch another circle at more than one point whether it touches it internally or externally..
Proposition 14.
Equal straight lines in a circle are equally distant from the center, and those which are equally distant from the center equal one another.
Proposition 15.
Of straight lines in a circle the diameter is greatest, and of the rest the nearer to the center is always greater than the more remote.
Proposition 16.
The straight line drawn at right angles to the diameter of a circle from its end will fall outside the circle, and into the space between the straight line and the circumference another straight line cannot be interposed, further the angle of the semicircle is greater, and the remaining angle less, than any acute rectilinear angle.