Corollary. From this it is manifest that the straight line drawn at right angles to the diameter of a circle from its end touches the circle.
Proposition 17.
From a given point to draw a straight line touching a given circle.
Proposition 18.
If a straight line touches a circle, and a straight line is joined from the center to the point of contact, the straight line so joined will be perpendicular to the tangent.
Proposition 19.
If a straight line touches a circle, and from the point of contact a straight line is drawn at right angles to the tangent, the center of the circle will be on the straight line so drawn.
Proposition 20.
In a circle the angle at the center is double the angle at the circumference when the angles have the same circumference as base.
Proposition 21.
In a circle the angles in the same segment equal one another.
Proposition 22.
The sum of the opposite angles of quadrilaterals in circles equals two right angles.
Proposition 23.
On the same straight line there cannot be constructed two similar and unequal segments of circles on the same side.
Proposition 24.
Similar segments of circles on equal straight lines equal one another.
Proposition 25.
Given a segment of a circle, to describe the complete circle of which it is a segment.
Proposition 26.
In equal circles equal angles stand on equal circumferences whether they stand at the centers or at the circumferences.
Proposition 27.
In equal circles angles standing on equal circumferences equal one another whether they stand at the centers or at the circumferences.
Proposition 28.
In equal circles equal straight lines cut off equal circumferences, the greater circumference equals the greater and the less equals the less.
Proposition 29.
In equal circles straight lines that cut off equal circumferences are equal.
Proposition 30.
To bisect a given circumference.
Proposition 31.
In a circle the angle in the semicircle is right, that in a greater segment less than a right angle, and that in a less segment greater than a right angle; further the angle of the greater segment is greater than a right angle, and the angle of the less segment is less than a right angle.
Proposition 32.
If a straight line touches a circle, and from the point of contact there is drawn across, in the circle, a straight line cutting the circle, then the angles which it makes with the tangent equal the angles in the alternate segments of the circle.
Proposition 33.
On a given straight line to describe a segment of a circle admitting an angle equal to a given rectilinear angle.
Proposition 34.
From a given circle to cut off a segment admitting an angle equal to a given rectilinear angle.
Proposition 35.
If in a circle two straight lines cut one another, then the rectangle contained by the segments of the one equals the rectangle contained by the segments of the other.
Proposition 36.
If a point is taken outside a circle and two straight lines fall from it on the circle, and if one of them cuts the circle and the other touches it, then the rectangle contained by the whole of the straight line which cuts the circle and the straight line intercepted on it outside between the point and the convex circumference equals the square on the tangent.
Proposition 37.
If a point is taken outside a circle and from the point there fall on the circle two straight lines, if one of them cuts the circle, and the other falls on it, and if further the rectangle contained by the whole of the straight line which cuts the circle and the straight line intercepted on it outside between the point and the convex circumference equals the square on the straight line which falls on the circle, then the straight line which falls on it touches the circle.