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the same multiple of AB, that LM is of CF.

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Again, be- Book V. cause LM is the same multiple of CF, that MN is of FD; therefore LM is the same multiplea of CF, that 1. 5. LN is of CD: But LM was shown to be the same multiple of CF, that GK is of AB; GK therefore is the same multiple of AB, that LN is of CD; that is, GK, LN are equimultiples of AB, CD. Next, because HK is the same multiple of EB, that MN is of FD; and that KX is also the same multiple of EB, that NP is of FD; X therefore HX is the same multiple of EB, that MP is of FD. And because AB is to BE, as CD is to DF, and that of AB and CD, GK and LN are equimultiples, and of EB and FD, HX and MP are equimultiples; if GK be greater than HX, then LN is greater than MP; and if equal, equal; and if less, less; But if GH be greater than KX, by adding the common part HK to both, GK is greater than HX; wherefore also LN is greater than MP; and by taking away MN from both, LM is greater than NP: Therefore, if GH be greater than KX, LM is greater than G A C L NP. In like manner it may be demonstrated, that if GH be equal to KX, LM likewise is equal to NP; and if less, less: And GH, LM are any equimultiples whatever of AE, CF, and KX, NP are any whatever of EB, FD. Therefore, as AE is to EB, so is CF to FD. If then magnitudes, &c. Q. E. D.

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PROP. XVIII. THEOR.

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If magnitudes, taken separately, be proportionals, See N. they shall also be proportionals when taken jointly, that is, if the first be to the second, as the third to the fourth, the first and second together shall be to the second, as the third and fourth together to the fourth.

Let AE, EB, CF, FD be proportionals; that is, as AE to EB, so is CF to FD; they shall also be proportionals when taken jointly; that is, as AB to BE, so CD to DF.

Take of AB, BE, CD, DF any equimultiples whatever GH, HK, LM, MN: and again, of BE, DF, take any whatever equimultiples KO, NP: And because KO, NP

BOOK V. are equimultiples of BE, DF; and that KH, NM are equimultiples likewise of BE, DF, if KO, the multiple of BE, be greater than KH, which is a multiple of the same BE, NP likewise the multiple of DF,

shall be greater than MN, the mul-
than MN, the mul

tiple of the same DF; and if KO 0-
be equal to KH, NP shall be equal
to NM; and if less, less..

First, let KO not be greater than
KH, therefore NP is not greater K
than NM: And because GH,
HK, are equimultiples of AB,
BE, and that AB is greater than
23 Ax. 5. BE, therefore GH is greatera

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than HK; but KO is not greater
than KH, wherefore GH is greater
than KO. In like manner it may
be shown, that LM is greater than
NP. Therefore, if KO be not greater
than KH, then GH, the multiple of AB, is always greater
than KO, the multiple of BE; and likewise LM, the mul-
tiple of CD, greater than NP, the multiple of DF.

Next, Let KO be greater than KH: therefore, as has been shown, NP is greater than NM: And because the whole GH is the same multiple of the whole AB, that HK is of BE, the remainder GK is the same multiple of the 5. 5. remainder AE that GH is of ABb:

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which is the same that LM is of 이 CD. In like manner, because LM is the same multiple of CD, that MN is of DF, the remainder LN is the same multiple of the remainder CF, that the whole LM K is of the whole CD: But it was shown that LM is the same multiple of CD, that GK is of AE; therefore GK is the same multiple of AE, that LN is of CF; that is, GK, LN are equimultiples of AE, G CF: And because KO, NP are equimultiples of BE, DF, if from KO, NP, there be taken. KH, NM, which are likewise equimultiples of BE, DF, the remainders HO, MP are either equal to DE, DF, or 6. 5. equimultiples of theme. First, let HO, MP be equal to BE, DF; and because AE is to EB, as CF to FD, and

A C L

d Cor. 4. 5.

that GK, LN are equimultiples of AE, CF; GK shall be Boox V. to EB, as LN to FDd: But HO is equal to EB, and MP to FD; wherefore GK is to HO, as LN to MP. If therefore GK be greater than HO, LN is greater than MP; and if equal, equal; and if less, less.

But let HO, MP be equimultiples of EB, FD; and because AE is to EB, as CF to FD, and that of AE, CF are taken equimultiples GK, LN; and of EB, FD, the equimultiples HO, MP; if GK be greater than HO, LÑ is greater than MP; and if equal, equal; and if less, lessf; which was likewise shown in the preceding

case.

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If therefore GH be greater than KO, taking KH from both, H GK is greater than HO; wherefore also LN is greater than MP; and consequently adding NM to both, LM is greater than NP: K Therefore, if GH be greater than KO, LM is greater than NP. In like manner it may be shown, that if GH be equal to KO, LM is equal to NP; and if less, less. And in the case in which KO is not greater than KH, it has been shown that GH is-always greater than KO, and likewise LM than NP: But GH, LM are any equimultiples of AB, CD, and KO, NP are any whatever of BE, DF; therefore, as AB is to BE, so is CD to DF. If then magnitudes, &c. Q. E.D.

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Ax. 5.

f 5 Def. 5.

PROP. XIX. THEOR.

Ir a whole magnitude be to a whole, as a magni- See N. tude taken from the first, is to a magnitude taken from the other; the remainder shall be to the remainder, as the whole to the whole.

Let the whole AB, be to the whole CD, as AE, a magnitude taken from AB, to CF, a magnitude taken from CD; the remainder EB shall be to the remainder FD, as the whole AB to the whole CD.

Because AB is to CD, as AE to CF; likewise, alternate

a 16.5. b 17.5.

BOOK V. lya, BA is to AE, as DC is to CF; and because
if magnitudes, taken jointly, be proportionals,
they are also proportionals b when taken sepa-
rately; therefore, as BE is to EA, so is DF to
FC, and alternately, as BE is to DF, so is EA to E
FC, But, as AE to CF, so by the hypothesis, is
AB to CD: therefore also BE, the remainder,
shall be to the remainder DF, as the whole AB
to the whole CD: Wherefore, if the whole, &c.
Q. E. D.

F

COR. If the whole be to the whole, as a mag- B D nitude taken from the first, is to a magnitude taken from the other; the remainder likewise is to the remainder; as the magnitude taken from the first to that taken from the other: The demonstration is contained in the preceding.

PROP. E. THEOR.

IF four magnitudes be proportionals, they are also proportionals by conversion: that is, the first is to its excess above the second, as the third to its excess above the fourth.

Let AB be to BE, as CD to DF; then BA is to AE, as DC to CF.

Because AB is to BE, as CD to DF, by di

* 17. 5. vision a, AE is to EB, as CF to FD; and by inB. 5. version, BE is to EA, as DF to FC. Where18. 5. fore, by composition, BA is to AE, as DC is to CF: If, therefore, four, &c. Q. E.D.

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See N. IF there be three magnitudes, and other three, which, taken two and two, have the same ratio; if the first be greater than the third, the fourth shall be greater than the sixth, and if equal, equal; and if less, less.

Let A, B, C, be three magnitudes, and D, E, F other three, Book V. which, taken two and two, have the same ratio, viz. as A is

to B, so is D to E; and as B to C, so is E to F. If A be greater than C, D shall be greater than F; and if equal, equal; and if less, less.

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a 8.5.

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Because A is greater than C, and B is any other magnitude, and that the greater has to the same magnitude a greater ratio than the less has to it; therefore A has to B a greater ratio than C has to B: But as D is to E, so is A to B; therefore D has to E a greater ratio than C to B; and because B is to C, as DE E to F, by inversion, C is to B, as F is to E; and D was shown to have to E a greater ratio than C to B; therefore D has to E a greater ratio than F to Ec. But the magnitude which has a greater ratio than another to the same magnitude, is the greater of the two; D is therefore greater than F. Secondly, Let A be equal to C; D shall be equal to F: Because A and C are equal to one another, A is to B, as C is to Be: But A is to B, as D to E; and C is to B, as F to E; wherefore D is to E, as F to Ef; and therefore D is equal to Fg. A Next, Let A be less than C; D shall be less than F: For C is D greater than A, and, as was shown in the first case, C is to B, as F to E, and in like manner, B is to A, as E to D; thereA,as fore F is greater than D, by the

Cor. 13.5.

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first case; and therefore D is less than F. Therefore, if there be three, &c. Q.E.D.

PROP. XXI. THEOR.

Ir there be three magnitudes, and other three, See N. which have the same ratio taken two and two, but in a cross order; if the first magnitude be greater than the third, the fourth shall be greater than the sixth; and if equal, equal; and if less, less.

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