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* 12. 2.

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* 4. 1.

* 8.6.

sequently the angle
BZK at the centre is
greater than a right
angle: and because
the angle BZK is
obtuse, the square

H
of BK is greater

SV2
than the squares of D
BZ, ZK; that is,
greater than twice
the
square

of BZ.
Join KV: and be-
cause in the triangles
KBV, OBV, KB,
BV, are equal to OB, BV, and that they contain equal
angles; the angle KVB is equal * to the angle OVB:
and OVB is a right angle; therefore also KVB
is a right angle: and because BD is less than twice
DV; the rectangle contained by BD, BV, is less than
twice the rectangle DVB; that is *, the square of KB
is less than twice the square of KV: but the square of
KB is greater than twice the square of BZ; therefore
the square of KV is greater than the square of BZ: and
because BA is equal to AK, and that the squares of
BZ, ZA are equal together to the square of BA, and
the squares of KV, VĂ to the square of AK; therefore
the squares of BZ, ZA are equal to the squares of KV,
VA; and of these the square of KV is greater than the
square of BZ; therefore the square of VA is less than
the square of ZA, and the straight line AZ greater
than ĎA: much more then is AZ greater than AG;
because, in the preceding proposition, it was shewn
that KV falls without the circle FGH: and AZ is
perpendicular to the plane KBOS, and is therefore the
shortest of all the straight lines that can be drawn from
A, the centre of the sphere, to that plane. Therefore
the plane KBOS does not meet the lesser sphere.

And that the other planes between the quadrants BX, KX, fall without the lesser sphere, is thus demonstrated. From the point A draw AI perpendicular to the plane of the quadrilateral SOPT, and join IO; and, as was demonstrated of the plane KBOS and the point Z, in the same way it may be shewn that the point I is the centre of a circle described about SOPT; and that OS is greater than PT; and PT was shewn to be

parallel to OS: therefore, because the two trapeziums KBOS, SOPT inscribed in circles have their sides BK, OS, parallel, as also OS, PT; and their other sides BO, KS, OP, ST, all equal to one another, and that BK is greater than OS, and OS greater than PT, therefore the straight line ZB is greater * than IO. *2Lem. 12. Join AO which will be equal to AB;

and because AIO, AZB are right angles, the squares of AI, 10 are equal to the square of AO or of AB; that is, to the squares of AZ, ZB; and the square of ZB is greater than the square of IO, therefore the square of AZ is less than the square of AI; and the straight line AZ less than the straight line AI: and it was proved, that AZ is greater than AG; much more then is Al greater than AG: therefore the plane SOPT falls wholly without the lesser sphere. In the same manner it may be demonstrated, that the plane TPRY falls without the same sphere, as also the triangle YRX, viz. by the Cor. of 2d Lemma. And after the same way it

may be demonstrated, that all the planes which contain the solid polyhedron, fall without the lesser sphere. Therefore in the greater of two spheres, which have the same centre, a solid polyhedron is inscribed, the superficies of which does not meet the lesser sphere. Which was to be done.

But the straight line AZ may be demonstrated to be greater than AG otherwise, and in a shorter manner, without the help of Prop. 16, as follows. From the point G draw GU at right angles to AG, and join AU. If then the circumference BE be bisected, and its half again bisected, and so on, there will at length be left a circumference less than the circumference which is subtended by a straight line equal to GU, inscribed in the circle BCDE: let this be the circumference KB: therefore the straight line KB is less than GU: and because the angle BZK is obtuse, as was proved in the preceding, therefore BK is greater than BZ: but GU is greater than BK; much more then is GU greater than BZ, and the square of GU than the square of BZ: and AU is equal to AB; therefore the square of AU, that is, the squares of AG, GU, are equal to the square of AB, that is, to the squares of AŻ, ZB: but the square of BZ is less than the square of GU; therefore the square of AZ is greater than the square of AG, and the straight line AŽ consequently greater than the straight line AG.

* B. 11.

COR. And if in the lesser sphere there be inscribed a solid polyhedron, by drawing straight lines betwixt the points in which the straight lines from the centre of the sphere drawn to all the angles of the solid polyhedron: in the greater sphere meet the superficies of the lesser; in the same order in which are joined the points in which the same lines from the centre meet the superficies of the greater sphere; the solid polyhedron in the sphere BCDE shall have to this other solid polyhedron the triplicate ratio of that which the diameter of the sphere BCDE has to the diameter of the other sphere. For if these two solids be divided into the same number of pyramids, and in the same order, the pyramids shall be similar to one another, each to each: because they have the solid angles at their common vertex, the centre of the sphere, the same in each pyramid, and their other solid angles at the bases, equal to one another, each to each *, because they are contained by three plane angles, each equal to each; and the pyramids are contained by the

same number of similar planes; and are therefore simi#11 Def.11. lar* to one another, each to each: but similar pyramids *Cor. 8.12. have to one another the triplicate * ratio of their homo

logous sides: therefore the pyramid of which the base is the quadrilateral KBOS, and vertex A, has to the pyramid in the other sphere of the same order, the triplicate ratio of their homologous sides, that is, of that ratio which AB from the centre of the greater sphere has to the straight line from the same centre to the superficies of the lesser sphere. And in like manner, each pyramid in the greater sphere has to each of the same order in the lesser, the triplicate ratio of that which AB has to the semi-diameter of the lesser sphere. And as one antecedent is to its consequent, so are all the antecedents to all the consequents. Wherefore the whole solid polyhedron in the greater sphere has to the whole solid polyhedron in the other, the triplicate ratio of that which AB the semidiameter of the first has to the semidiameter of the other ; that is, which the diameter BD of the greater has to the diameter of the other sphere.

PROP. XVIII. THEOR.

Spheres have to one another the triplicate ratio of that

which their diameters have.

Let ABC, DEF, be two spheres, of which the diameters are BC, EF. The sphere ABC shall have to the sphere DEF the triplicate ratio of that which BC has to EF.

For, if it has not, the sphere ABC must have to a sphere either less or greater than DEF, the triplicate ratio of that which BC has to EF. First, let it have that ratio to a less, viz. to the sphere GHK; and let the sphere DEF have the same centre with GHK: and in

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the greater sphere DEF inscribe* a solid polyhedron, *17. 12. the superficies of which does not meet the lesser sphere GHK; and in the sphere ABC inscribe another similar to that in the sphere DEF: therefore the solid polyhedron in the sphere ABC has to the solid polyhedron in the sphere DEF, the triplicate ratio * of that Cor. 17. which BC has to EF. But the sphere ABC has to the 12. sphere GHK, the triplicate ratio of that which BC has to EF; therefore as the sphere ABC to the sphere GHK, so is the solid polyhedron in the sphere ABC to the solid polyhedron in the sphere DEF: but the sphere ABC is greater than the solid polyhedron in it; therefore * also the sphere GHK is greater than the solid * 14. 5. polyhedron in the sphere DEF: but it is also less, because it is contained within it, which is impossible : therefore the sphere ABC has not to any sphere less than DEF, the triplicate ratio of that which BC has to EF. In the same manner it may be demonstrated, that the sphere DEF has not to any sphere less than ABC, the triplicate ratio of that which EF has to BC. Nor can the sphere ABC have to any sphere greater than DEF, the triplicate ratio of that which BC has to EF: for, if it can, let it have that ratio to a greater sphere LMN: therefore, by inversion, the sphere LMN has to the sphere ABC, the triplicate ratio of that

which the diameter EF has to the diameter BC. But as the sphere LMN to ABC, so is the sphere DEF to some sphere, which must be less * than the sphere ABC, because the sphere LMN is greater than the sphere DEF; therefore the sphere DEF has to a sphere less than ABC the triplicate ratio of that which EF has to BC; which was shewn to be impossible: therefore the sphere ABC has not to any sphere greater than DEF the triplicate ratio of that which BC has to EF: and it was demonstrated, that neither has it that ratio to any sphere less than DEF. Therefore the sphere ABC has to the sphere DEF, the triplicate ratio of that which BC has to EF.

* 14. 5.

Q. E. D.

END OF THE ELEMENTS.

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