PROP. XIV. THEOR.

If the first have to the second the same ratio which the third has to the fourth, and if the first be greater than the third, the second shall be greater than the fourth; if equal, equal; and if less, less.

If A: B:: C: D; then if A7C, B7D; if A=C, B=D; and if ALC, BLD.

First, let A7C; then A: B7C: B (8. 5.), but A: B:: C: D, therefore C: D7C: B (13. 5.), and therefore B7D (10.5.).

In the same manner, it is proved, that if A=C,`B=D; and if ALC, BLD. Therefore, &c. Q. E. D.

PROP. XV. THEOR.

Magnitudes have the same ratio to one another which their equimultiples have.

If A and B be two magnitudes, and m any number, A: B:: mA: mB.

Because A: B: A: B (7. 5.); A: B:: A+A: B+B (12. 5.), or A: B: 2A: 2B. And in the same manner since A: B:: 2A: 2B, A : B : : A+2A::B+2B (12. 5.), or A : B :: 3A : 3B; and so on, for all the equimultiples of A and B. Therefore, &e. Q. E. D.

PROP. XVI. THEOR.

If four magnitudes of the same kind be proportionals, they will also be proportionals when taken alternately.

If A: B:: C: D, then alternately, A: C:: B: D.

Take mA, mB any equimultiples of A and B, and nC, nD any equimultiples of C and D). Then (15. 5.) A: B:: mA : mB; now_A: B::CD, therefore (11. 5.) C: D:: mA: mB. But CD:: nCnD (15. 5.); therefore mA: mB:: nCnD (11. 5.): wherefore if mA7nC, mB7nD (14.5.); if mA=nC, mB=nD, or if mAL nC, mB▲nD; therefore (def. 5. 5.) A: C:: B : D. Therefore, &c. Q. E. D.

PROP. XVII. THEOR.

If magnitudes, taken jointly, be proportionals, they will also be proportionals when taken separately; that is, if the first, together with the second, have to the second the same ratio which the third, together with the fourth, has to the fourth, the first will have to the second the same ratio which the third has to the fourth.

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

Take mA and nB any multiples of A and B, by the numbers m and n; and first let mA7nB: to each of them add mB, then mA+mB7 mB+uB. But mA+mBm(A+B) (Cor. 1. 5.), and mB+nB= (m+n)B (2. Cor. 2. 5.), therefore m(A+B7 (m+n) B.

And because A+B: BC+D: D, if m(A+B)7(m+n) B, m(C+D)7(m+n)D, or m€+m7mD÷nD, that is taking mD from both, mC7n. Therefore, when mA is greater than nB, mC is greater than nD. In like manner, it is demonstrated, that if mA=nB, mC=nD, and if mAnB, that mDnD; 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 proportionals 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: BC: D, then, by composition, A+B : BC+D: D. Take m(A+B), and nB any multiples whatever of A+B and B: aud first, let m 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(CD) is greater than nD. And in the same manner it may be proved, that when m=n,m(A+B) is greater than nB, and m (C÷D) greater than nD.

Next, let mn, orn 7, 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 mA7nB—mB, or mA7(n—m)B(6.5.). ButifmA7(n--m) B.mC7 (n-m)D, because A: BCD. Now, (nm)=nD— mD (6.5.), therefore, mC7nD-mD, and adding mD to both, mC+m DnD, that is (1.5.),m(C+i))7n D. If therefore, m(A+B) 7nB, m(C+D)7nD.

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

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 remainder as the whole to the whole.

If A: BC: D, and if C be less than A, A-C: B-D: A: B. Because A: BC: 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-DC: D; but A: B: C: D, there

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

PROP. D. 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.

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 (18. 5.), À : A—B : : C: C-D. Therefore, &c. Q. E. D. Q.E.D. COR. In the same way, it may be proved that A: A+B :: C : C+D.

PROP. XX. THEOR.

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

:: EF, then if A7C, D7F; if A=C,

D-F: and if AC, DF.

A, B, C,

D, E, F,

First, let A7C; then A: B7C: B (8. 5.). But A: B D: E, therefore also D: É7C:B(13.5.). Now B:CE:F, and inversely (A. 5.), C: B:: F: E; and it has been shown 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: E, therefore, D): E::F: E (11.5.), and D-F (9.5.). Lastly, let AC. Then C7A, and because, as was already shown, C: B:: F: E, and B: A:: E: D; therefore, by the first case, if C7A, F7D, that is, if AC, DF. Therefore, &c. Q.E. D.

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 if less, less.

If there be three magnitudes, A, B, C, and other three, D, E, and F, such that A : B :: E; F, and BC:: D:E: if A7C, D7F:ff A=C, D F, and if AC, D¿F.

A, B, C,
D, E, F.

First, let A7C. Then A: B7C: B (8. 5.), but A: BE: F, therefore E: F7C: B (13. 5.). Now, B: C:: D: E, and inversely, C:BE: 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, CB:: E: D, therefore (11. 5.), E: F:: E: D, and, consequently, D=F (9. 5.).

Lastly, let ALC. Then C7A, and, as was already proved, C: BED; and B: A:: F: E, therefore, by the first case, since C7A, F7D, that is, DF. Therefore, &c. Q. E. D.

PROP. XXII. THEOR:

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.*

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, nE: and of C and F any whatever, qC, qF. Because AB::D: E, mA : nB::mD:nE (4. 5.); and for the same reason, nB : qC : : nE qF. Therefore (20. 5.), according as mA is greater than qC. equal to it, or less, mD is great. er than qF, equal to it or less; but mA, mD are any equimultiples of A and D; and qC. qF are any and F; therefore (def. 5. 5:), A : C :: D : F.

A, B, C, D, E, F, mA, nB, qC, mD, nE, qF.] equimultiples of C

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: C:: 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 and two, have the same ratio, by the foregoing case, A: C::E: G.

And because also

A, B, C, D.

E. F. G, H,

In the same

CD::G: H, by that same case, A: D:: E: H. manner is the demonstration extended to any number of magnitudes. Therefore, &c. Q. E. D.

*N. B. This proposition is usually cited by the words "ex aequali,” or “ex aequo,”

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 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.*

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, have 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, nE, nF.

Because A: 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 : : mD :

:

A, B, C, D, E, F, mA, mB, nC,

mD, nE nF.

nE (4. 5.); and it has been just shown that mA: mB:: nE: nF; therefore, if mA7nC, mD7nF (21. 5.); if mA=nC, mD=nF; and if mAnC, mDnF. Now, mA and mD are any equimultiples of A and D, and nC, nF, any equimultiples of C and F; therefore, A: C:: D: F (def. 5. 5.).

A,

B, C, D.

E,

F.

F, G, H,

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: H. For, since A, B, C, are three magnitudes, 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, by the first case, A: D:: EH. In the same manner, may the demonstration be extended to any number of magnitudes. Therefore, &c. Q. E. D.

PROP. XXIV. THEOR.

If the first has to the second the same ratio which the third has to the fourth; and the fifth to the second, the same 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 hypothesis, A: B:: C: D, therefore, ex æquali (22. 5.), A:E::C:F; and by composition (18. 5.), 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.

N. B. This proposition is usually cited by the words "ex aequali in proportione pertur bata:" or, "ex aequo inversely."

Q