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c Cor.

Book V. Let A, B, C be three magnitudes, and D, E, F other three,

which have the same ratio, taken two and two, but in a cross
order, viz. as A is to B, so is E to F, and as B
is to C, so is D to E. If A be greater than C,
D shall be greater than F; and if equal, equal ;
and if less, less.

Because A is greater than C, and B is any a 8. 5. other magnitude, A has to B a greater ratio a

than C has to B: but as E to F, so is A to B; 13. 5. therefore b E has to F a greater ratio than C to

A B с B: and because B is to C, as D to E, by inversion, C is to B, as E to D: and E was shown to D E F have to F a greater ratio than C to B; there

fore E has to F a greater ratio than E to Dc; 13. 5.

but the magnitude to which the same has a

greater ratio than it has to another, is the lesser d 10. 5. of the twod; F therefore is less than D; that is,

Dis greater than F.

Secondly, Let A be equal to C; D shall be equal to F. Bee 7.5. cause A and C are equal, A ise to B, as C is to B: but A is

to B, as E to F; and C is to B

as E to D; wherefore E is to F f 11. 5.

as E to Df; and therefore D is
equal to F 8.

Next, Let A be less than C;
D shall be less than F: for C is
greater than A, and, as
shown, C is to B, as E to D, A B С A

с and in like manner B is to A,

D E F D E F as F to E; therefore F is greater than D, by case first; and therefore D is less than F. Therefore, if there be three, &c. Q. E. D.

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PROP. XXII. THEOR.

See N.

IF there be any number of magnitudes, and as many others, which, taken two and two in order, have the same ratio; the first shall have to the last of the first magnitudes the same ratio which the first of the others has to the last. N. B. This is usually cited by the words ex æquali,or ex æquo.

First, Let there be three magnitudes A, B, C, and as many Book V. others D, E, F, which, taken two and two, have the same ratio, that is, such that A is to B as D to E; and as B is to C, so is E to F; A shall be to C, as D to F.

Take of A and D any equimultiples whatever G and H; and of B and E any equimultiples whatever K and L; and of C and F any whatever M and N: then, because A is to B, as D to E, and that G, H are equimultiples of A, D, and K, L equimultiples of B, A

B

D E F E; as G is to K, so is a H to L. G км H L N a 4. 5: For the same reason, K is to M, as L to N: and because there' are three magnitudes G, K, M, and other three H, L, N, which, two and two, have the same ratio; if G be greater than M, H is greater than N; and if equal, equal ; and if less, less b; and G, H are

b 20. 5. any equimultiples whatever of A, D, and M, N are any equimultiples whatever of C, F. Therefore c, as A is to C, so is D c 5. def. to F.

5. Next, let there be four magnitudes Ą, B, C, D, and other four E, F, G, H, which two and two have the same ratio, viz. as A is to B, so is E to F, and A. B. C. D. as B to C, so F to G; and as C to D, so G to E. F. G. H. H: A shall be to D, as E to H.

Because 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 is to C, as E to G. But C is to D, as G is to H; wherefore again, by the first case, A is to D, as E to H: and so on, whatever be the number of laagnitudes. Therefore, if there be any number, &c. Q. E. D.

Book v.

PROP. XXIII. THEOR.

See N.

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 shall have to the last of the first magnitudes the same ratio which the first of the others has to the last. N. B. This is usu. ally cited by the words ex equali in proportione per. turbata ;or, ex æquo perturbatc.

First, Let there be three magnitudes A, B, C, and other three D, E, F, which, taken two and two, in a cross order, have the same ratio, that is, such that A is to B, as E to F; and as B is to C, so is D to E: A is to C, as D to F.

Take of A, B, D any equimultiples whatever G, H, K; and of C, E, F any equimultiples whatever L, M, N; and because G, H are equimultiples of A, B, and that magnitudes have the

same ratio which their equimula 15. 5. tiples have a; as A is to B, so is

G to H. And, for the same rea-
son, as E is to F, so is M to N:
but as A is to B, so is E to F; A B с D F

as therefore G is to H, so is M to G H L K M N b 11. 5. Nb. And because as B is to C,

so is D to E, and that H, K are

equimultiples of B, D, and L, M C 4. 5.

of C, E; as H is to L, so is © K
to M: and it has been shown,
that G is to H, as M to N: then,
because there are three magni-
tudes G, H, L, and other three
K, M, N, which have the same
ratio taken two and two in a cross

order; if G be greater than 1, d 21. 5. K is greater than N; and if equal, equal; and if less, less d;

and G, K are any equimultiples whatever of A, D; and L, N any whatever of C, F; as, therefore, A is to C, so is D to F.

Next, Let there be four magnitudes, A, B, C, D, and other Book V. four E, F, G, H, which taken two and two in a cross order have the same ratio, viz.

A. B. C. D. A to B, as G to H; B to C, as F to G;

E. F. G. H. and C to D, as E to F: A is to D as E

to H.

Because 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 is to C, as F to H: but C is to D, as E is to F; wherefore again, by the first case, A is to D, as E to H: and so on, whatever be the number of magnitudes. Therefore, if there be any number, &c. Q. E. D.

PROP. XXIV. THEOR.

IF the first has to the second the same ratio which See N. 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.

H

Let AB the first have to C the second the same ratio which DE the third has to F the fourth ; and let BG the fifth have to C the second the same ratio which EH

G the sixth has to F the fourth : AG, the first and fifth together, shall have to C the second the same ratio which DH, the third and sixth together, has to F the

Bfourth.

E! Because BG is to C, as EH to F; by inversion, C is to BG, as F to EH : and because, as AB is to C, so is DE to F; and

a 22. 5. as C to BG, so F to EH ; er æquali a, AB is to BG, as DE to EH: and because these magnitudes are proportionals, they shall likewise be proportionals when taken A jointly b: as, therefore, AG is to GB, so

b 18. 5. is DH to HE; but as GB 10 C, so is HE to F. Therefore, er æquali a, as AG is to C, so is DH to F. Wherefore, if the first, &c. Q. E. D.

Cor. 1. If the same hypothesis be made as in the proposition, the excess of the first and fifth shall be to the second, as

Book V. the excess of the third and sixth to the fourth. The demonstra

tion 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 ihat 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 proportionats, viz.

AB to CD, as E to F; and let AB be the greatest of them, and a A. & consequently P the least a AB, together with F, are greater 14. 5.

than CD, together with E.

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.

B And because AB the whole is to the whole CD, as AG is to CH, likewise the

G D remainder GB shall be to the remainder b 19. 5. HD, as the whole AB is to the whole b

H н
CD: but AB is greater than CĎ, there-
c A. 5. forec GB is greater than HD: and be-

cause 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 A с Е F
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.

PROP. F. THEOR.

See N.

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

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