the angle BAE equal to the angle ABD, and produce Book III BD, if necessary, to E, and join EC; and because the

E

f

E

AC

h 9. 3.

angle ABE is equal to the angle BAE, the straight line BE is equal to EA: and because AD is equal to DC, f6.1. and DE common to the triangles ADE, CDE, the two sides AD, DE are equal to the two CD, DE, each to each; and the angle ADE is equal to the angle CDE, for each of them is a right angle; therefore the base AE is equal to the base EC: but AE was shewn to be equal g 4. 1. to EB, wherefore also BE is equal to EC: and the three straight lines AE, EB, EC are therefore equal to one another; wherefore E is the centre of the circle. From the centre E, at the distance of any of the three AE, EB, EC describe a circle, this shall pass through the other points; and thus the circle of which ABC is a segment is described: also, it is evident, that if the angle ABD be greater than the angle BAD, the centre E falls without the segment ABC, which therefore is less than a semicircle: but if the angle ABD be less than BAD, the cen tre E falls within the segment ABC, which is therefore greater than a semicircle: Wherefore, a segment of a circle being given, the circle is described of which it is a segment. Which was to be done.

Book III.

PROP. XXVI. THEOR.

In equal circles, equal angles stand upon equal arches, whether they be at the centres or circumferences.

Let ABC, DEF be equal circles, and the equal angles BGC, EHF at their centres, and BAC, EDF at their circumferences: the arch BKC is equal to the arch ELF.

Join BC, EF; and because the circles ABC, DEF are equal, the straight lines drawn from their centres are equal therefore the two sides BG, GC, are equal to the A

a 4. 1.

c 24. 3.

b

two EH, HF; and the angle at G is equal to the angle at H; therefore the base BC is equal a to the base EF: and because the angle at A is equal to the angle at D, b 9. def. 3. the segment BAC is similar to the segment EDF; and they are upon equal straight lines BC, EF; but similar segments of circles upon equal straight lines are equal e to one another; therefore the segment BAC is equal to the segment EDF: but the whole circle ABC is equal to the whole DEF; therefore the remaining segment BKC is equal to the remaining segment ELF, and the arch BKC to the arch ELF. Wherefore in equal circles, &c. Q. E. D.

Book III.

PROP. XXVII. THEOR.

In equal circles, the angles which stand upon equal arches are equal to one another, whether they be at the centres or circumferences.

Let the anglés BGC, EHF at the centres, and BÁC, EDF at the circumferences of the equal circles ABC, DEF stand upon the equal arches BC, EF: the angle BGC is equal to the angle EHF, and the angle BAC to the angle EDF.

If the angle BGC be equal to the angle EHF, it is manifesta that the angle BAC is also equal to EDF. a 20. 3. But, if not, one of them is the greater: let BGC be the greater, and at the point G, in the straight line BG, make the angle BGK equal to the angle EHF. And ь 23. 1. because equal angles stand upon equal arches, when c 26. 3. they are at the centre, the arch BK is equal to the arch EF: but EF is equal to BC; therefore also BK is equal to BC, the less to the greater, which is impossible. Therefore the angle BGC is not unequal to the angle

EHF; that is, it is equal to it: and the angle at A is half of the angle BGC, and the angle at D half of the angle EHF therefore the angle at A is equal to the angle at D. Wherefore, in equal circles, &c. Q. E. D.

Book III.

a 1. 3.

PROP. XXVIII. THEOR.

In equal circles, equal straight lines cut off equal arches, the greater equal to the greater, and the less to the less.

Let ABC, DEF be equal circles, and BC, EF equal straight lines in them, which cut off the two greater arches BAC, EDF, and the two less BGC, EHF: the greater BAC is equal to the greater EDF, and the less BGC to the less EHF.

Take a K, L, the centres of the circles, and join BK, KC, EL, LF: and because the circles are equal, the

b 8. 1.

€ 26. 3.

b

e

straight lines from their centres are equal; therefore BK, KC are equal to EL, LF; but the base BC is also equal to the base EF; therefore the angle BKC is equal to the angle ELF: and equal angles stand upon equal arches, when they are at the centres; therefore the arch BGC is equal to the arch EHF. But the whole circle ABC is equal to the whole EDF the remaining part, therefore, of the circumference, viz. BAC, is equal to the remaining part EDF. Therefore, in equal circles, &c. Q. E. D.

Book III.

PROP. XXIX. THEOR.

In equal circles equal arches are subtended by equal straight lines.

Let ABC, DEF be equal circles, and let the arches BGC, EHF also be equal; and join BC, EF: the straight line BC is equal to the straight line EF.

a

Take K, L the centres of the circles, and join BK, a 1. 3. KC, EL, LF: and because the arch BGC is equal to the arch EHF, the angle BKC is equal to the angle & 27.3. ELF: also because the circles ABC, DEF are equal, their radii are equal: therefore BK, KC are equal to

EL, LF; and they contain equal angles: therefore the base BC is equal to the base EF. Therefore, in equal e 4. F. circles, &c. Q. E. D.

PROP. XXX. PROB.

To bisect a given arch, that is, to divide it into two equal parts.

it.

Let ADB be the given arch; it is required to bisect

Join AB, and bisect a it in C; from the point C draw a 10: 1. CD at right angles to AB, and join AD, DB: the arch

ADB is bisected in the point D.