## Euclid's Elements, Book III

 elem.3.1 To find the centre of a given circle. f. 048 digilib elem.3.2 If on the circumference of a circle two points be taken at random, the straight line joining the points will fall within the circle. f. 048 digilib elem.3.3 If in a circle a straight line through the centre bisect a straight line not through the centre, it also cuts it at right angles; and if it cut it at right angles, it also bisects it. f. 049 digilib elem.3.4 If in a circle two straight lines cut one another which are not through the centre, they do not bisect one another. f. 050 digilib elem.3.5 If two circles cut one another, they will not have the same centre. f. 050 digilib elem.3.6 If two circles touch one another, they will not have the same centre. f. 050 digilib elem.3.7 If on the diameter of a circle a point be taken which is not the centre of the circle, and from the point straight lines fall upon the circle, that will be greatest on which the centre is, the remainder of the same diameter will be least, and of the rest the nearer to the straight line through the centre is always greater than the more remote, and only two equal straight lines will fall from the point on the circle, one on each side of the least straight line. f. 050 digilib elem.3.8 If a point be taken outside a circle and from the point straight lines be drawn through to the circle, one of which is through the centre and the others are drawn at random, then, of the straight lines which fall on the concave circumference, that through the centre is greatest, while of the rest the nearer to that through the centre is always greater than the more remote, but, of the straight lines falling on the convex circumference, that between the point and the diameter is least, while of the rest the nearer to the least is always less than the more remote, and only two equal straight lines will fall on the circle from the point, one on each side of the least. f. 051 digilib elem.3.9 If a point be taken within a circle, and more than two equal straight lines fall from the point on the circle, the point taken is the centre of the circle. f. 053 digilib elem.3.10 A circle does not cut a circle at more points than two. f. 053 digilib elem.3.11 If two circles touch one another internally, and their centres be taken, the straight line joining their centres, if it be also produced, will fall on the point of contact of the circles. f. 054 digilib elem.3.12 If two circles touch one another externally, the straight line joining their centres will pass through the point of contact. f. 055 digilib elem.3.13 A circle does not touch a circle at more points than one, whether it touch it internally or externally. f. 055 digilib elem.3.14 In a circle equal straight lines are equally distant from the centre, and those which are equally distant from the centre are equal to one another. f. 056 digilib elem.3.15 Of straight lines in a circle the diameter is greatest, and of the rest the nearer to the centre is always greater than the more remote. f. 056 digilib elem.3.16 The straight line drawn at right angles to the diameter of a circle from its extremity 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 rectilineal angle. f. 057 digilib elem.3.17 From a given point to draw a straight line touching a given circle. f. 058 digilib elem.3.18 If a straight line touch a circle, and a straight line be joined from the centre to the point of contact, the straight line so joined will be perpendicular to the tangent. f. 058 digilib elem.3.19 If a straight line touch a circle, and from the point of contact a straight line be drawn at right angles to the tangent, the centre of the circle will be on the straight line so drawn. f. 059 digilib elem.3.20 In a circle the angle at the centre is double of the angle at the circumference, when the angles have the same circumference as base. f. 059 digilib elem.3.21 In a circle the angles in the same segment are equal to one another. f. 059 digilib elem.3.22 The opposite angles of quadrilaterals in circles are equal to two right angles. f. 060 digilib elem.3.23 On the same straight line there cannot be constructed two similar and unequal segments of circles on the same side. f. 060 digilib elem.3.24 Similar segments of circles on equal straight lines are equal to one another. f. 060 digilib elem.3.25 Given a segment of a circle, to describe the complete circle of which it is a segment. f. 061 digilib elem.3.26 In equal circles equal angles stand on equal circumferences, whether they stand at the centres or at the circumferences. f. 061 digilib elem.3.27 In equal circles angles standing on equal circumferences are equal to one another, whether they stand at the centres or at the circumferences. f. 062 digilib elem.3.28 In equal circles equal straight lines cut off equal circumferences, the greater equal to the greater and the less to the less. f. 063 digilib elem.3.29 In equal circles equal circumferences are subtended by equal straight lines. f. 063 digilib elem.3.30 To bisect a given circumference. f. 063 digilib elem.3.31 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; and further the angle of the greater segment is greater than a right angle, and the angle of the less segment less than a right angle. f. 064 digilib elem.3.32 If a straight line touch a circle, and from the point of contact there be drawn across, in the circle, a straight line cutting the circle, the angles which it makes with the tangent will be equal to the angles in the alternate segments of the circle. f. 065 digilib elem.3.33 On a given straight line to describe a segment of a circle admitting an angle equal to a given rectilineal angle. f. 065 digilib elem.3.34 From a given circle to cut off a segment admitting an angle equal to a given rectilineal angle. f. 067 digilib elem.3.35 If in a circle two straight lines cut one another, the rectangle contained by the segments of the one is equal to the rectangle contained by the segments of the other. f. 067 digilib elem.3.36 If a point be taken outside a circle and from it there fall on the circle two straight lines, and if one of them cut the circle and the other touch it, 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 will be equal to the square on the tangent. f. 068 digilib elem.3.37 If a point be taken outside a circle and from the point there fall on the circle two straight lines, if one of them cut the circle, and the other fall 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 be equal to the square on the straight line which falls on the circle, the straight line which falls on it will touch the circle. f. 069 digilib

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