*

fore as C is to D, so is G to H: but it was proved that as C is to D, so is E to F, therefore, as E is to F, so is G to H. But when four magnitudes are proportionals *, if the first be greater than the third, the second is greater than the fourth; and if equal, equal; if less, less; therefore if E be greater than G, F likewise is greater than H; and if equal, equal; if less, less: and E, F are any † equimultiples whatever of A, B ; and

15. 5.

11. 5.

14.5.

Constr.

G, H any whatever of C, D: therefore *, A is to C as B to D. * 5 Def. 5. If then, four magnitudes, &c. Q. E. D.

PROPOSITION XVII.

THEOR.-If magnitudes, taken jointly, be proportionals, they See N. shall also be proportionals when taken separately; that is, if two magnitudes together, have to one of them, the same ratio which two others have to one of these, the remaining one of the first two shall have to the other, the same ratio which the remaining one of the last two has to the other of these.

Let AB, BE, CD, DF be the magnitudes, taken jointly, which are proportionals; that is, as AB to BE, so let CD bet to DF: they shall also be proportionals taken separately; viz. as AE to EB, so shall CF be to FD.

Take of AE, EB, CF, FD, any equimultiples whatever GH, HK, LM, MN; and again, of EB, FD, take any equimultiples whatever KX, NP: and because GH is the same multiple of AE, that HK is of EB, therefore GH is the same multiple of AE, that GK is of AB: but GH is the same mul- * 1. 5. tiple of AE, that LM is of CF; therefore GK is the same multiple of AB, that LM is of CF. Again, because LM is the same multiple of CF, that MN is of FD, therefore LM is the same multiple of CF, that LN is of CD: but LM was shewn to be the same multiple of CF, that GK is of AB; therefore GK 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, therefore HX is the

*

*

K

HB

P

N

* 1. 5.

M

EF

GACL

same multiple of EB, that MP is of FD. And because AB * 2. 5. is to BE +, as CD is to DF, and that of AB and CD, GK and + Hyp.

#5 Def. 5.

† 4 Ax. 1.

† 5 Ax. 1.

+ Constr.

* 5 Def. 5.

See N.

LN are equimultiples, and of EB and FD, HX and MP are
equimultiples, therefore, 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, then, 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 NP.
In like manner it may be demonstrated,
that if GH be equal to KX, LM is equal to NP ; and if less, less:
but 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.

PROPOSITION XVIII.

THEOR.-If magnitudes, taken separately, be proportionals, 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 let CF be to FD: they shall also be proportionals when taken jointly; that is, as AB to BE, so shall CD be to DF.

II

O

Take of AB, BE, CD, DF, any equimultiples whatever GH, HK, LM, MN; and again, of BE, DF, take any equimultiples whatever KO, NP: and because KO, NP are equimultiples of BE, DF, and that KH, NM are likewise equimultiples of BE, DF, therefore if KO, the multiple of BE, be greater than KH, which is a multiple of the same BE, then NP, the multiple of DF, is also greater than NM, the multiple of the same DF; and if KO be equal to KH, NP is equal to NM; and if less, less.

K

G

M

P

N

N

First, let KO be not greater than KH; therefore NP is not greater than NM and because GH, HK are equimultiples of AB, BE, and that AB is greater than BE, therefore GH is * 3 Ax. 5. greater than HK ; but KO is not greater than KH; therefore

GH is greater than KO. In like manner it may be shewn, 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 multiple of
CD, is greater than NP, the multiple of DF.

K

* 5. 5.

P

M

* 5.5.

Next, let KO be greater than KH, therefore, as has been shewn, NP is greater than NM and because the whole GH is the same multiple of the whole AB, that HK is of BE, therefore the remainder GK is the same multiple of the remainder AE that GH is of AB; which is the same that LM is of CD. In like manner, because LM is the same multiple of CD, that MN is of DF, therefore the remainder LN is the same multiple of the remainder CF*, that the whole LM is of the whole CD: but it was shewn, 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, CF. And because KO, NP are equimultiples of BE, DF, therefore if from KO, NP there be taken KH, NM, which are likewise equimultiples of BE, DF, the remainders HO, MP are either equal to BE, DF, or equimultiples of them*. First, * 6. 5. let HO, MP be equal to BE, DF: then because † AE is to EB, † Hyp. as CF to FD, and that GK, LN are equimultiples of AE, CF, therefore GK is to EB*, as LN to FD: but HO is equal to EB, * Cor. 4. 5. and MP to FD; wherefore GK is to HO, as LN to MP: there

fore if GK be greater than HO, LN is greater than * MP; and * A. 5. if equal, equal; and if less, less.

H

5 Def. 5.

But let HO, MP be equimultiples of EB, FD: then †, be- + Hyp. cause 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, LN is greater than MP; and if equal, equal; and if less, less*: which was likewise shewn in the preceding case. But if GH be greater than KO, taking KH from both, GK is greater than HO; wherefore also LN is greater than MP; and consequently, adding NM to both, LM is greater † than NP therefore, if GH be greater than KO, LM is greater than NP. ner it may be shewn, 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 shewn that GH is always greater than KO, and likewise LM greater than NP: but GH, LM are any equimultiples

In like man

† 5 Ax. 1.

P

M

† 4 Ax. 1.

E

KB

+ Constr.

* 5 Def. 5.

See N.

16. 5.

* 17.5.

† 11.5.

* 17. 5. * B. 5.

* 18.5.

whatever 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.

PROPOSITION XIX.

THEOR.-If a whole magnitude be to a whole, as a magnitude 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, is to CF a magnitude taken from CD: the remainder EB shall be to the remainder FD, as the whole AB to the whole CD.

E

с

F

Because AB is to CD, as AE to CF, therefore, alternately *, BA is to AE, as DC to CF: and because, if magnitudes taken jointly be proportionals, they are also proportionals when taken separately, therefore, as BE is to EA, so is DF to FC; and alternately, as BE is to DF, so is EA to FC: but as AE to CF, so, by the hypothesis, is AB to CD; therefore also, BF the remainder is to the remainder DF †, as the whole AB to the whole CD. Wherefore, if the whole, &c. Q. E. D.

B D

COR. If the whole be to the whole, as a magnitude taken from the first, is to a magnitude taken from the other, the remainder shall likewise be to the remainder, as the magnitude taken from the first to that taken from the other. The demonstration is contained in the preceding.

PROPOSITION 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 shall be to AE, as DC to CF.

Because AB is to BE, as CD to DF, therefore by division*, AE is to EB, as CF to FD; and by inversion*, BE is to EA, as DF to FC; wherefore, by composition *, BA is to AE, as DC is to CF. If, therefore, four, &c. Q. E. D.

E

C

B D

PROPOSITION XX.

THEOR.-If there be three magnitudes, and other three, See N. which, taken two and two, have the same ratio, then 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, 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.

8.5.

+ Hyp. 13. 5.

A BC B. 5.
DEF

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 Et, so is A to B; therefore* D has to E a greater ratio than C to B: and because B is to C, as E to F, by inversion †, C is to B, as F is to E and D was shewn to have to E, a greater ratio than C to B; therefore D has to E a greater ratio than F to E: but the magnitude which has a greater ratio than another to the same magnitude, is the greater of the two; therefore D is 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 B*: 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 E*; and therefore D is equal to F*.

Next, let A be less than C: D shall be less than F. For C is greater than A; and,

as was shewn in the first case, C is to B,

A BC

DEFA

as F to E; and in like manner, B is to A, as E to D; therefore F is greater than D, by the first case; that is, D is less than F. Therefore, if there be three, &c.

PROPOSITION XXI.

Q. E. D.

• Cor. 13.5.

⚫ 10. 5.

• 7.5.

+ Hyp.
+ Hyp. &
B. 5.

11. 5.

* 9.5.

THEOR.-If there be three magnitudes, and other three, which See N. have the same ratio taken two and two, but in a cross order, then 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.