Take mA and nB any multiples of A and B, by the numbers m and n; and first let mA 7nB : to each of them add inB, then in A+mB7 mB+nB. But mA+mB=m(A+B) (Cor. 1. 5.), and mB+nB= (m+n)B (2. Cor. 2. 5.), therefore m(A+B7m+n)B.

And because A+B: B :: C+D :D, of m(A+B)7(m+n) B, m(C+D)7(m+n)D, or mC+mD 7mnD+nD, that is taking mD from both, mc7nD. Therefore, when mA is greater than nB, mc is greater than nD. ln like manner,it is demonstrated that if mA=nB, mC=nD, and if mA ZnB, that mDZnD; therefore A: B::C: D (def. 5.5.). Therefore, &c. · Q. E. D.

PROP. XVIII. THEOR.

If magnitudes, taken separately, be proportionals, they will also be pro

portionals when taken jointly, that is, if the first be to the second as the third to the fourth, the first and second together will be to the second as the third and fourth together to the fourth.

If A: B::C:D, then, by composition, A+B:B :: C+D : D.

Take m(A+B), and nB any multiples whatever of A+B and B: and first, let in be greater than n. Then, because A+B is also greater than B, m(A+B) 7nB. For the same reason, m(C+D) 7nD. In this case, therefore, that is, when m7n,m(A+B) is greater than nB, and m(C+D) is greater than nD. And in the same manner it may be proved, that when mun,i(A+B) is greater than nB, and m (C+D) greater than nD.

Next, let man, or n7m, then m(A+B) may be greater than nB, or may be equal to it, or may be less ; first, let m(A+B) be greater than nB; then also, mA+mB7nB; take mB, which is less than nB, from both, and mA 7nB-mB, or mA 7(

nn)B(6.5). But if mA7(n- m) B, C7 (n--m)D, because A : B::C:D Now, (n-m)D=nDmD (6. 5.), therefore, mC7nD-mD, and adding mD to both, mC+m D7nD, that is; (1, 5.),m(C+D)7n.D. If therefore, míA+B) 7nB, m(C+D7nD.

In the same manner it will be proved, that if m(A+B)=nB,m(C+ D=nD; and if m(A+B) ZnB,m(C+D) ZnD; therefore (def.5.5.), AUB: B::C+D: D. Therefore, &c. Q. E. D.

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PROP. 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 will be to the ren mainder as the whole to the whole.

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If A:B::C:D, and if C be less than A, A-C:B-D::A: B.

Because A : B ::C:D, alternately (16. 5.), A :C:: B:D; and therefore by division (17. 5.) A-C: C::B-D: D. Wherefore, again alternately, A-C:B_D::C: D; but A : B:: C :D, there

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fore (11. 5.) A.C:B-D::A: B. Therefore, &c. Q. E. D.

Cor. A-C:B-D:-:C:D.

PROP. D. THEOR.

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If four magnitudes be proportionals, they are also proportionals by con. version, that is, the first is to its excess above the second, as the third to its excess above the fourth. If A :B::C:D, by conversion, A : A-B ::C:C-D.

For, since A:B::C:D, by division (17. 5.), A-B:B::C-D D, and inversely (A. 5.), B : A-B ::D:C-D; therefore, by composition (!8. 5), A: A-B::C:C-D. Therefore, &c. Q. E. D.

Cor. In the same way, it may be proved that A : A+B ::C: C+D.

PROP. XX. THEOR.

C,

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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 is greater than the sixth; if equal, equal; and if less, less. .

If there be three magnitudes, A, B, and C, and other three D, E, and F; and if A:B :: D:E; and also B :C A, B, ::E:F, then if A7C, D7F; if A=C, D, E, F, D=F: and if ALC, DLF. First, let A7C ; then A : B7C: B (8. 5.). But A : B::D: E,

; therefore also D : E7C:B (13.5.). Now B: C::E: F,and inversely (A 5.), C:B::F: E; and it has been shewn that D: E7C:B, therefore D: E7F:E (13. 5.), and consequently D7F (10. 5.). Next, let A=C; then A :B :: C:B (7.5.), but A : B :: D: E;

; therefore, C:B::D: E, but C B::F: É, therefore, D :E F: E (11.5.), and DEF (9. 5.). Lastly, let A ZC. Then C7 A, and because, as was already shewn, C:B::F: E, and B :A:: E:D; therefore, by the first case, if C7 A, F7D, that is, if ALC, D4F. Therefore, &c. Q E. D.

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PROP. XXI.. THEOR.

If there be three magnitudes, and other three, 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 is greater than the sixth; if equal, equal ; and f less, less.

If there be three magnitudes, A, B, C, and other three, D, E, and F, such that A : B : :E: F, and B:C::D:E : A7C, D7F; if A=C, D=F, and if ALC, D4F.

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First, let A 7C. Then A:B7C: B (8.6.),

A, B, C, but A:B: :E:F, therefore E: F7C :B

D, E, F. (13.5). Now, B : C::D: E, and inversely, C: B : :E:D; therefore, E : F7E:D (13. 5.), wherefore, D7F (10. 5.).

Next, let A=C. Then (7.5.) A: B::C:B; but A : B :: E:F, therefore, C:B::E: F (11. 5.) ; but B: C::D: E, and inversely, 0 : B: : E : D, therefore (11. 5.), E:F::E:D, and, consequently, D=F (9. 5.).

Lastly, let AZC. Then C7 A, and, as was already proved, C:B:: E:D ; and B: A::F: E, therefore, by the first case, since CZA F7D, that is, D_F. Therefore, &c. Q. E. D.

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If there be any number of magnitudes, and as many others, which, taken

two and two in order, have the same ratio ; the first will have to the last of the first magnitudes, the same ratio which the first of the others has to the last.*

E.

EB
B.

First, let there be three magnitudes, A, B, C, and other three, D, E, F, which, taken two and two, in order, have the same ratio, viz. A:B::D: E, and B:C::E:F; then A :C::D:F.

Take of A and D any equimultiples whatever, mA, mD; and of B and D any whatever, nB, nĒ: and of C and F any whatever, qC,qF. Because A :B::D: E, MA : nB : : mD :nE

A, B, C, (4. 5.); and for the same reason, nB : qC :: nE:

D, E,

E, F, gF. Therefore (20. 5.), according as mA is

mA, nB, qC, greater than qC, equal to it, or less, mD is great

mD, nE gF. er than qF, equal to it or less ; but mA, mĎ are any equimultiples of A and D; and qC, 9F are any equimultiples of C and F; therefore (def. 5. 5.), A : C::D:F.

' Again, let there be four magnitudes, and other four which, taken two and two in order, have the same ratio, viz. A :B::E:F;B: €::F:G;C:D::G: H, then A :D::E: H.

For since A, B, C are three magnitudes, and E, F, G other three, which, taken two

A, B, C, D, and two have the same ratio, by the forego

E, F, G, H, ing case, A:C::E: G. And because also C:D::G : H, by that same case, A :D :: E: H. In the same manner is the demonstration extended to any number of magnitudes. Therefore, &c. Q. E. D.

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* N. B. This proposition is usually cited by the words “ ex æguali," or "ex æguer"

Q

PROP. XXIII. THEOR.

If there be any number of magnitudes, and as many others, which, taken

two and two, in a cross order, have the same rotio ; the first will have to the last of the first magnitudes the same ratio which the first of the others has to the last.*

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First, let there be three magnitudes, A, B, C, and other three, D, E, and F, which, taken two and two in a cross order, bave the same ratio, viz. A :B::E:F, and B:C::D: E, then A:C::D:F. Take of A, B. and D, any equimultiples mA, mB, mD; and of C, E, F any equimultiples nC, nt, nF.

Because À : B::E: F, and because also A : B::MA : mB (15.5.), and E:F:: nE : nF ; therefore, mA : mB: : nE : nF (11. 5.). Again, because B:C: D:E, mB : nC : :D :

A, B, C, NE (4. 6.); and it has been just shewn that mA :

D, E, F, mB: : nE: nF ; therefore if mA ZnC, mD 7 nF

mA, mB, nc, (21. 5.); if mA=nC, mD=nF; and if maZnC, mD2nF. Now, mA and mD are any equimulti

mD, nE, nF. ples of A and D, and nC, nF any equimultiples of C and F; there

'; fore, A:C::D:F (def. 5.5.).

Next, Let there be four magnitudes, A, B, C, and D, and other four, E, F, G, and H, which taken two and two, in a cross order, have the same ratio, viz. A:B::G:H; B :C:: F: G, and C:D::E:F, then A :D::E: A, BC, D, H. For, since A, B, C, are three magnitudes, F, G, and F, G, H other three, which taken two and two, in a cross order, have the same ratio, by the first case, A :C:: F: H. But C D::E:F, therefore, again, hy the first case, A: D: : E:H. In the same manner, may the demonstration be extend. ed to any number of magnitudes. Therefore, &c. Q. E. D.

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H.

If the first has to the second the same ratio which the third has to the fourth;

and the fifth to the second, the saine ratio which the sixth has to the fourth ; the first and fifth, together, shall have to the second, the same ratio which the third and sixth together, have to the fourth.

Let A : B::C:D, and also E : B::F :D, then A+E:B :: C+F : D.

Because E :B ::F:D, by inversion, B :E ::D:F. But by hy. pothesis, A:B::C: D, therefore, ex æquali (22,5.), A:E::C: F: and by composition (18. b.), A+E: E::C+F:F. And again by hypothesis, E: B::F:D, therefore, ex æquali (22. 5.), A+E: B::C+F :D Therefore &c. Q. E. D.

PROP. E. THEOR.

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If four magnitudes be proportionals, the sum of the first two is to their

difference as the sum of the other two to their difference. Let A:B::C:D; then if A7B,

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A4.B:A-B::C+D:C-D; or if ALB

A+B:B-A::C+D: D-C.
For, if A7B, then because A:B::C:D, by division (17. 5:),

A-B:B::C-D:D, and by inversion (A. 5.),
B:A-B ::D:C-D. But, by composition (18. 5.),
A+B:B::C+D:D, therefore, ex æquali (22.5.),

A+B: A-B::C+D:C-D.
In the same manner, if B7A, it is proved, that

A+B:B-A::C+D:D-C. Therefore, &c.
Q. E. D.

PROP. F. THEOR.

Ratios which are compounded of equal ratios, are equal to one another.

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Let the ratios of A to B, and of B to C, which compound the ratio of A to C, be equal, each to each, to the ratios of D to E, and E to F, which compound the ratio of D to F, A :C::D:F. For, first, if the ratio of A to B be equal to

A, B,

B, C, that of D to E, and the ratio of B to C equal to

D, E, F. that of E to F, ex æquali (22. 5.), A:C::D:F.

And next, if the ratio of A to B be equal to that of E to F, and the ratio of B to C equal to that of D to È, ex æquali inversely (23. 5.), A : C::D:F. In the same manner may the proposition be demonstrated, whatever be the number of ratios. Therefore, &c. Q. E. D.