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Let A be to B as D to E; and B to Cas 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

A. B. C. compound ratio, is the ratio of A to C, is

D. E. F. 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.

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 22. 5. Next, Let A be to B as E to F, and B to C as D to E; therefore, ex æquali in proportione perturbatab, A is

23. 5. to C as D to F; that is, the ratio of A to C,

A. B. C. which is compounded of the ratios of A to B,

D. E. F. 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.

PROP. G. THEOR.

IF several ratios be the same with several ratios, See N. each to each; the ratio which is compounded of ra. tios 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.

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,

A. B. C. D. K. L. M. which are the same with the ra

E. F. G. H. N. O. P. tios of A to B, and C to D: and as E to F, so let N be to 0; 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 0); 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 æquali a, K is to M as N to P. Therefore, if several

ratios, &c. Q. E. D. a 22. 5.

PROP. H. THEOR.

See N.

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.

com.

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 same with the ratio of G to M, which

A. B. C. D. E. F. is compounded of the other rapounded tios : and besides, let the ratio of A

G. H. K. L. M. ratio.

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 10K:
then the ratio compounded of the remaining first ratios, to wit, of
the ratios of D to Ē, 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 b, 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 10 X is compounded

h, k, l. A, B; C, D; E, F. G, H; K, L; M, N; 0, 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 do 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 I be that which is compounded of the ratios of h to k, and k tol, 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 I, as m to p.

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Because e is to f, as (G to H, that is, as) Y to Z; and f is 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 æquali, 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 I, as (E to F, that is, as) V to X; therefore, ex æquali, h is to I, as T to X: in like manner, it

may

be demonstrated, that m is to p, as a tod: and it has been shown, that T a 11. 5. is to X, as a to d; therefore a h is to I, as m to p. Q. E. D.

The propositions G and K are usually, for the sake of brevily, 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.

THE

ELEMENTS OF EUCLID.

BOOK VI.

DEFINITIONS.

I. SIMILAR rectilineal figures

Book VI. are those which have their several angles equal, each to each, and the sides about the equal angles proportionals.

II. Reciprocal figures, viz. triangles and parallelograms, are such See N. « as have their sides about two of their angles proportionals “ in such manner, that a side of the first figure is to a side “ of the other, as the remaining side of this other is to the re« maining side of the first.”

III.
A straight line is said to be cut in extreme and mean ratio, when

the whole is to the greater segment, as the greater segment is
to the less.

IV. The altitude of any figure is the straight line drawn from its vertex perpendicular to the base.

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