Book V. the excess of the third and sixth to the fourth. The demonstration of this is the same with that of the proposition, if division be used instead of composition.

COR. 2. The proposition holds true of two ranks of magnitudes, whatever be their number, of which each of the first rank has to the second magnitude the same ratio that the corresponding one of the second rank has to a fourth magnitude; as is manifest.

PROP. XXV. THEOR.

IF four magnitudes of the same kind are proportionals, the greatest and least of them together are greater than the other two together.

Let the four magnitudes AB, CD, E, F be proportionals, viz. AB to CD, as E to F; and let AB be the greatest of them, and a A. & consequently F the least 2. AB, together with F, are greater than CD, together with E.

14. 5.

B

G D

Take AG equal to E, and CH equal to F: then, because as AB is to CD, so is E to F, and that AG is equal to E, and CH equal to F; AB is to CD, as AG to CH. And because AB the whole is to the whole CD, as AG is to CH, likewise the remainder GB shall be to the remainder b 19. 5. HD, as the whole AB is to the whole b CD: but AB is greater than CD, therec A. 5. fore GB is greater than HD and because AG is equal to E, and CH to F ; AG and F together are equal to CH and E together. If, therefore, to the unequal magnitudes GB, HD, of which GB is the greater, there be added equal magnitudes, viz. to GB the two AG and F, and CH and E to HD; AB and F together are greater than CD and E. Therefore, if four magnitudes, &c, Q. E. D.

E F

See N.

PROP. F. THEOR.

RATIOS which are compounded of the same ratios, are the same with one another.

A. B. C.

Let A be to B as D to E; and B to C as E to F: the ratio which Book V. is compounded of the ratio of A to B, and B to C, which, by the definition of compound ratio, is the ratio of A to C, is the same with the ratio of D to F, which, by the same definition, is compounded of the ratios of D to E, and E to F.

D. E. F.

Because there are three magnitudes A, B, C, and three others D, E, F, which, taken two and two in order, have the same ratio; ex æquali, A is to C as D to Fa.

A. B. C.

D. E. F.

Next, Let A be to B as E to F, and B to C as D to E; therefore, ex æquali in proportione perturbata, A is to C as D to F; that is, the ratio of A to C, which is compounded of the ratios of A to B, and B to C, is the same with the ratio of D to F, which is compounded of the ratios of D to E, and E to F and in like manner the proposition may be demonstrated, whatever be the number of ratios in either case.

a 22. 5.

b 23. 5.

PROP. G. THEOR.

IF several ratios be the same with several ratios, See N. each to each; the ratio which is compounded of ratios which are the same with the first ratios, each to each, is the same with the ratio compounded of ratios which are the same with the other ratios, each to each.

A. B. C. D.
E. F. G. H.

K. L. M.
N. O. P.

Let A be to B as E to F; and C to D as G to H: and let A be to B as K to L; and C to D as L to M: then the ratio of K to M, by the definition of compound ratio, is compounded of the ratios of K to L, and L to M, which are the same with the ratios of A to B, and C to D: and as E to F, so let N be to O; and as G to H, so let O be to P; then the ratio of N to P is compounded of the ratios of N to O, and O to P, which are the same with the ratios of E to F, and G to H: and it is to be shown that the ratio of K to M is the same with the ratio of N to P, or that K is to M as N to P.

Because K is to L as (A to B, that is, as E to F, that is, as) N to O; and as L to M, so is (C to D, and so is G to H, and so is)

Book V. O to P: ex æqualia, K is to M as N to P. Therefore, if several ratios, &c. Q. E. D.

a 22. 5.

See N.

PROP. H. THEOR.

IF a ratio compounded of several ratios be the same with a ratio compounded of any other ratios, and if one of the first ratios, or a ratio compounded of any of the first, be the same with one of the last ratios, or with the ratio compounded of any of the last; then the ratio compounded of the remaining ratios of the first, or the remaining ratio of the first, if but one remain, is the same with the ratio compounded of those remaining of the last, or with the remaining ratio of the last.

Let the first ratios be those of A to B, B to C, C to D, D to E, and E to F; and let the other ratios be those of G to H, H to K, K to L, and L to M : also, let the ratio of A to F, which is coma Defini-pounded of a the first ratios, be the

tion of com.

ratio.

A. B. C. D. E. F.
G. H. K. L. M.

same with the ratio of G to M, which is compounded of the other rapounded tios: and besides, let the ratio of A to D, which is compounded of the ratios of A to B, B to C, C to D, be the same with the ratio of G to K, which is compounded of the ratios of G to H, and H to K: then the ratio compounded of the remaining first ratios, to wit, of the ratios of D to E, and E to F, which compounded ratio is the ratio of D to F, is the same with the ratio of K to M, which is compounded of the remaining ratios of K to L, and L to M of the other ratios.

b B. 5.

Because, by the hypothesis, A is to D as G to K, by inversion ↳, c 22.5. D is to A as K to G; and as A is to F, so is G to M; therefore, ex æquali, D is to F as K to M. If, therefore, a ratio which is, &c. Q. E. D.

Book V.

PROP. K. THEOR.

IF there be any number of ratios, and any number See N. of other ratios such, that the ratio compounded of ratios which are the same with the first ratios, each to each, is the same with the ratio compounded of ratios which are the same, each to each, with the last ratios; and if one of the first ratios, or the ratio which is compounded of ratios which are the same with several of the first ratios, each to each, be the same with one of the last ratios, or with the ratio compounded of ratios which are the same, each to each, with several of the last ratios: then the ratio compounded of ratios which are the same with the remaining ratios of the first, each to each, or the remaining ratio of the first, if but one remain; is the same with the ratio compounded of ratios which are the same with those remaining of the last, each to each, or with the remaining ratio of the last.

Let the ratios of A to B, C to D, E to F, be the first ratios; and the ratios of G to H, K to L, M to N, O to P, Q to R, be the other ratios: and let A be to B, as S to T; and C to D, as T to V; and E to F, as V to X: therefore, by the definition of compound ratio, the ratio of S to X is compounded

h, k, l. A, B; C, D; E, F. G, H; K, L, M, N, O, P; Q, R. e, f, g. m, n, o, p.

S, T, V, X,
Y, Z, a, b, c, d.

of the ratios of S to T, T to V, and V to X, which are the same with the ratios of A to B, C to D, E to F, each to each; also, as G to H, so let Y be to Z; and K to L, as Z to a; M to N, as a to b, O to P, as b to c; and Q to R, as c to d therefore, by the same definition, the ratio of Y to d is com pounded of the ratios of Y to Z, Z to a, a to b, b to c, and

U

Book V. c to d, which are the same, each to each, with the ratios of G to H, K to L, M to N, O to P, and Q to R: therefore, by the hypothesis, S is to X, as Y to d: also, let the ratio of A to B, that is, the ratio of S to T, which is one of the first ratios, be the same with the ratio of e to g, which is compounded of the ratios of e to f, and f to g, which, by the hypothesis, are the same with the ratios of G to H, and K to L, two of the other ratios; and let the ratio of h to 1 be that which is compounded of the ratios of h to k, and k to 1, which are the same with the remaining first ratios, viz. of C to D, and E to F; also, let the ratio of m to p be that which is compounded of the ratios of m to n, n to o, and o to p, which are the same, each to each, with the remaining other ratios, viz. of M to N, O to P, and Q to R: then the ratio of h to 1 is the same with the ratio of m to p, or h is to l, as m to p.

h, k, l.

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

G, H; K, L; M, N, O, P; Q, R.
e, f, g.

m, n, o, p.

S, T, V, X.
Y, Z, a, b, c, d.

Because e is to f, as (G to H, that is, as) Y to Z; and fis to g, as (K to L, that is, as) Z to a; therefore, ex æquali, e is to g, as Y to a and by the hypothesis, A is to B, that is, S to T, as e to g; wherefore S is to T, as Y to a; and, by inversion, T is to S, as a to Y; and S is to X, as Y to d; therefore, ex aquali, T is to X, as a to d: also, because h is to k, as (C to D, that is, as) T to V; and k is to 1, as (E to F, that is, as) V to X; therefore, ex aquali, h is to 1, as T to X: in like manner, it may be demonstrated, that m is to p, as a to d: and it has been shown, that T a 11. 5. is to X, as a to d; therefore a h is to 1, as m to p. Q. E. D. The propositions G and K are usually, for the sake of brevity, expressed in the same terms with propositions F and H: and therefore it was proper to show the true meaning of them when they are so expressed; especially since they are very frequently made use of by geometers.