€ 23. 1.

d 4. 1.

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Book III. shortest line FD at the point E, in the straight line EF, make the angle FEH equal to the angle GEF, and join FH: then because GE is equal to EH, and EF common to the two triangles GEF, HEF; the two sides GE, EF are equal to the two HE, EF; and the angle GEF is equal to the angle HEF; therefore the base FG is equald to the base FH: but, besides FH, no other straight line can be drawn from F to the circumference equal to FG: for, if there can, let it be FK; and because FK is equal to FG, and FG to FH, FK is equal to FH; that is, a line nearer to that which passes through the centre, is equal to one which is more remote; which is impossible. Therefore, if any point be taken, &c. Q. E. D.

PROP. VIII. THEOR.

IF any point be taken without a circle, and straight lines be drawn from it to the circumference, whereof one passes through the centre, of those which fall upon the concave circumference, the greatest is that which passes through the centre, and, of the rest, that which is nearer to that through the centre is always greater than the more remote: but of those which fall upon the convex circumference, the least is that between the point without the circle and the diameter; and, of the rest, that which is nearer to the least is always less than the more remote: and only two equal straight lines can be drawn from the point unto the circumference, one upon each side of the least.

Let ABC be a circle, and D any point without it, from which let the straight lines DA, DE, DF, DC be drawn to the circumference, whereof DA passes through the centre. Of those which fall upon the concave part of the circumference AEFC, the greatest is AD which passes through the centre; and the nearer to it is always greater than the more remote, viz. DE than DF, and DF than DC; but of those which fall upon the Convex circumference HLKG, the least is DG between the

point D and the diameter AG; and the nearer to it is always Book III less than the more remote, viz. DK than DL, and DL than

DH.

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2

c

H

D

LK GBN

€ 24. 1.

M

d 4. Ax.

Take M the centre of the circle ABC, and join ME, MF a 1. 3. MC, MK, ML, MH: and because AM is equal to ME, add MD to each, therefore AD is equal to EM, MD; but EM, MD are greater than ED; therefore also AD is greater than ED: 6b 20. 1. again, because ME is equal to MF, and MD common to the triangles EMD, FMD; EM, MD are equal to FM, MD; but the angle EMD is greater than the angle FMD; therefore the base ED is greater than the base FD: in like manner it may be shown, that FD is greater that CD: therefore DA is the greatest: and DE greater than DF, and DF than DC and because MK, KD are greater than MD, and MK is equal to MG, the remainder KD is greater than the remainder C GD, that is, GD is less than KD: and because MK, DK are drawn to the point K within the triangle MLD from M, D, the extremities of its side MD; MK, KD are less than ML, LD, whereof MK is equal to ML; therefore the remainder DK is less than the remainder DL: in like manner it may be shown, that DL is less than DH: therefore DG is the least, and DK less than DL, and DL than DH: also there can be drawn only two equal straight lines from the point D to the circumference, one upon each side of the least: at the point M, in the straight line MD, make the angle DMB equal to the angle DMK, and join DB: and because MK is equal to MB, and MD common to the triangles KMD, BMD, the two sides KM, MD are equal to the two BM, MD; and the angle KMD is equal to the angle BMD; f4.1. therefore the base DK is equal to the base DB: but, besides DB, there can be no straight line drawn from D to the circumference equal to DK: for if there can, let it be DN; and because DK is equal to DN, and also to DB; therefore DB is equal to DN, that is, the nearer to the least equal to the more remote, which is impossible. If, therefore, any point, &c, Q. E. D.

F

E

A

e 21. 1.

Book 111.

a 7.3.

& 9.3.

PROP. IX. THEOR.

IF a point be taken within a circle, from which there fall more than two equal straight lines to the circumference, that point is the centre of the circle.

Let the point D be taken within the circle ABC, from which to the circumference there fall more than two equal straight lines, viz. DA, DB, DC; the point D is the centre of

the circle.

For, if not, let E be the centre, joir DE, and produce it to the circumference in F, G; then FG is a diameter of the circle ABC: and because in FG, the diameter of the F circie ABC, there is taken the point D which is not the centre, DG shall be the greatest line from it to the circumference, and DC greater athan DB, and DB than DA; but they are likewise equal, which is impossible: therefore E is not the centre of the circle ABC: in like manner it may be demonstrated, that no other point but D is the centre; D therefore is the centre. Wherefore, if a point be taken, &c. Q. E. D.

PROP. X. THEOR

ONE circumference of a circle cannot cut another in more than two points.

the centre of the circle DEF: but K is also the centre of the Book III. circle ABC; therefore the same point is the centre of two circles that cut one another, which is impossible. Therefore b 5. 3. one circumference of a circle cannot cut another in more than two points. Q. E. D.

PROP. XI. THEOR.

IF two circles touch each other internally, the straight line which joins their centres being produced shall pass through the point of contact.

Let the two circles, ABC, ADE touch each other internally in the point A, and let F be the centre of the circle ABC, and G the centre of the circle ADE: the straight line which joins the centres F, G, being produced, passes through the point A.

'H

D

A

a 20. 1.

G F

C

For, if not, let it fall otherwise, if possible, as FGDH, and join AF, AG: and because AG, GF are greater a than FA, that is, than FH, for FA is equal to FH, both being from the same centre; take away the common part FG; therefore the remainder AG is greater than the remainder GH:` but AG is equal to GD; therefore GD is greater than GH, the less than the greater, which is impossible. Therefore the straight line which joins the points F, G cannot fall otherwise than upon the point A, that is, it must pass through it. Therefore, if two circles, &c. Q. E. D.

PROP. XII. THEOR.

B

IF two circles touch each other externally, the straight line which joins their centres shall pass through the point of contact.`

Let the two circles ABC, ADE touch each other externally in the point A ; and let F be the centre of the circle ABC, and G the centre of ADE: the straight line which join the points F, G shall pass through the point of contact A.

For, if not, let it pass otherwise, if possible, as FCDG, and

Book III. join FA, AG: and because F is the centre of the circle ABC,

a 20.1.

E

possible: therefore the straight line which joins the points F, G shall not pass otherwise than through the point of contact A, that is, it must pass through it. Therefore, if two circles, &c. Q. E. D.

See Note.

PROP. XIII. THEOR.

ONE circle cannot touch another in more points than one, whether it touches it on the inside or outside.

For if it be possible, let the circle EBF touch the circle ABC in more points than one, and first on the inside, in the points a 10. 11. 1. B, D; join BD, and draw a GH bisecting BD at right angles: Therefore, because the points B, D are in the circumference

b 2.3.

of each of the circles, the straight line BD falls within each e cor. 1. 3. of them: and their centres are in the straight line GH which bisects BD at right angles; therefore GH passes through the point of contact; but it does not pass through it, because the points B, Dare without the straight line GH, which is absurd : therefore one circle cannot touch another on the inside in more points than one.

d 11. 3.