equal to F. Divide AB into magnitudes equal to E, viz. AG, GB; and CD into CH, HD equal each of them to F: the number therefore of the magnitudes CH, HD shall be equal to the number of the others, AG, GB; and because AG is equal to E, and CH to F, therefore AG and CH together are equal to (Ax. 2. 5.) E and F together for the same reason, because GB is equal to E, and HD to F; GB and HD together are equal to E and F together. Wherefore, as many magnitudes as are in AB equal to E, so many are there in AB, CD together equal to E and F together. Therefore, whatsoever multiple AB is of E, the same multiple is AB and CD together of E and F together.

Therefore, if any magnitudes, how many soever, be equimultiples of as many, each of each, whatsoever multiple any one of them is of its part, the same multitple shall all the first magnitudes be of all the other: For the same demonstration holds in any number of ⚫ magnitudes which was here applied to two.' Q. E. D.

PROP. II. THEOR.

Ir the first magnitude be the same multiple of the second that the third is of the fourth, and the fifth the same multiple of the second that the sixth is of the fourth; then shall the first together with the fifth be the same multiple of the second, that the third together with the sixth is of the fourth.

Α

D

Let AB the first, be the same multiple of C the second, that DE the third is of F the fourth; and BG the fifth, the same multiple of C the second, that EH the sixth is of F the fourth: then is AG the first, together with the fifth, the same multiple of C the second, that DH the third, together with the sixth, is of F the fourth.

Because AB is the same multiple of C, that DE is of F; there are as many magnitudes in AB equal to C, as there are in DE equal to F: in like manner, as many as there

B

ET

F

H

G

are in BG equal to C, so many are there in EH equal to F: as many, then, as are in the whole AG equal to C, so many are there in the whole DH equal to F: therefore AG is the same multiple

13

THE ELEMENTS OF EUCLID.

B

K

G

H

PROP. III. THEOR.

Ir the first be the same multiple of the second, which the third is of the fourth; and if of the first and third there be taken equimultiples, these shall be equimultiples, the one of the second, and the other of the fourth.

Let A the first, be the same multiple of B the second, that C the third is of D the fourth; and of A, C let the equimultiples EF, GH be taken; then EF is the same multiple of B that GH is of D.

Because EF is the same multiple of A that GH is of C, there are as many magnitudes in EF equal to A; as are in GH equal to C: let EF be divided into the magnitudes

K

EK, KF, each equal to A, and GH F into GL, LH, each equal to C: the number therefore of the magnitudes EK, KF, shall be equal to the number of the others GL, LH: and because A is the same multiple of B, that C is of D, and that EK is equal to A, and GL to C; therefore EK is the same multiple of B, that GL is of D; for the same reason KF is the same multiple of B, that LH is of D; and so, if there be more parts in EF, GH equal to A, C: because, therefore,

E

H

L

G

the first EK is the same multiple of the second B, which the third GL is of the fourth D, and that the fifth KF is the same multiple of the second B, which the sixth LH is of the fourth D; EF the first together with the fifth, is the same multiple (2. 5.) of the second B, which GH the third, together with the sixth, is of the fourth D. If, therefore, the first, &c. Q. E. D.

PROP. IV. THEOR.

Ir the first of four magnitudes has the same ratio to the second which the third has to the fourth, then any equimultiples whatever of the first and third shall have the same ratio to any equimultiples of the second and fourth, viz. the equimultiple of the 'first shall have the same ratio to that of the second, which the 'equimultiple of the third has to that of the fourth.'*

Let A the first, have to B the second, the same ratio which the third C has to the fourth D; and of A and C let there be taken any equimultiples whatever E, F: and of B and D any equimultiples whatever G, H: then E has the same

ratio to G, which F has to H.

Take of E and F any equimultiples whatever K, L, and of G, H, any equimultiples whatever M, N: then, because E is the same multiple of A, that F is of C; and of E and F have been taken equimultiples K, L; therefore K is the same multiple of A, that L is of C (3. 5.); for the same reason, M is the same multiple of B, that N is of D: and because, as A is to B, so is C to D (Hypoth.) and of A and C have been taken certain equimultiples K, L; and of B and D have been taken certain equimultiples M, N; if therefore K be greater than M, L is greater than N; and if equal, equal; if less, less (5. def. 5.). And K, L are any equimultiples whatever of E, F; and M, N any whatever of G, H: as therefore E is to G, so is (5. def. 5.) F to H. Therefore, if the first, &c. Q. E. D.

COR. Likewise, if the first have the same ratio to the second, which the third has to the fourth, then also any

equimultiples whatever of the first and third have the same ratio to the second and fourth: and in like manner, the first and the third have the same ratio to any equimultiples whatever of the second and fourth.

Let A the first, have to B the second, the same ratio which the third C has to the fourth D, and of A and C let E and F be any equimultiples whatever; then E is to B, as F to D.

Take of E, F any equimultiples whatever K, L, and of B, D any equimultiples whatever G, H; then it may be demonstrated, as be

• See Note.

།

THE ELEMENTS OF EUCLID.

BOOK V.

fore, that K is the same multiple of A, that L is of C: and because A is to B, as C is to D, and of A and C certain equimultiples have been taken, viz. K and L; and of B and D certain equimultiples G, H; therefore if K be greater than G, L is greater than H; and if equal, equal; if less, less (5. def. 5.): and K, L are any equimultiples of E, F, and G, H any whatever of B, D: as therefore E is to B, so is F to D: and in the same way is the other case demonstrated.

PROP. V. THEOR.

Ir one magnitude be the same multiple of another, which a magnitude taken from the first is of a magnitude taken from the other; the remainder shall be the same multiple of the remainder, that the whole is of the whole.

Let the magnitude AB be the same multiple of CD, that AE taken from the first, is of CF taken from the other; the remainder EB shall be the same multiple of the remainder FD, that the whole AB is of the whole CD.

G

A

E

F

Take AG the same multiple of FD, that AE is of CF: therefore AE is (1. 5.) the same multiple of CF, that EG is of CD; but AE, by the hypothesis, is the same multiple of CF that AB is of CD, therefore EG is the same multiple of CD that AB is of CD; wherefore EG is equal to AB (1. Ax. 5.). Take from them the common magnitude AE; the remainder AG is equal to the remainder EB. Wherefore, since AE is the same multiple of CF, that AG is of FD, and that AG is equal to EB; therefore AE is the same multiple of CF, that EB is of FD: but AE is the same multiple of CF, that AB is of CD; therefore EB is the same multiple of FD, that AB is of CD. Therefore, if any magnitude, &c. Q. E. D.

PROP. VI. THEOR.

B

D

Ir two magnitudes be equimultiples of two others, and if equimultiples of these be taken from the first two, the remainders are either equal to these others, or equimultiples of them.*

Let the two magnitudes AB, CD be equimultiples of the two E, F, and AG, CH taken from the first two be equimultiples of the same E, F; the remainders GB, HD are either equal to E, F, or equimultiples of them.

* See Note.

First, let GB be equal to E, HD is equal to F: make CK equal to F; and because AG is the same multiple of E, that CH is of F, and that GB is equal to E, and CK to F; therefore AB is the same multiple of E, that KH is of F. But AB, by the hypothesis, is the same multiple of E that CD is of F; therefore KH is the same multiple of F, that CD is of F; wherefore KH is equal to CD (1. Ax. 5.): take away the common magnitude CH, then the remainder KC is equal to the remainder HD; but KC is equal to F; HD therefore is equal to F.

G

K

A

H

But let GB be a multiple of E: then HD is the same multiple of F: make CK the same multiple of F, that GB is of E: and because AG is the same multiple of E, that GH is of F; and GB the same multiple of E that CK is of F; therefore AB is the same multiple of E, that KH is of F (2.5.): but AB is the same multiple of E, that CD is of F, therefore KH is the same multiple of F, that CD is of it: wherefore KH is equal to CD (1. Ax. 5.) : take away CH from both: therefore the remainder KC is equal to the remainder HD: and because GB is the same multiple of E, that KC is of F, and that KC is equal to HD; therefore HD is the same multiple of F, that GB is of E. If therefore two magnitudes, &c. Q. E. D.

PROP. A. THEOR.

B DE F

Ir the first of four magnitudes have to the second the same ratio which the third has to the fourth; then, if the first be greater than the second, the third is also greater than the fourth; and if equal, equal; if less, less.*

Take any equimultiples of each of them, as the doubles of each; then, by def. 5th of this book, if the double of the first be greater than the double of the second, the double of the third is greater than the double of the fourth; but if the first be greater than the second, the double of the first is greater than the double of the second; wherefore also the double of the third is greater than the double of the fourth; therefore the third is greater than the fourth: in like manner, if the first be equal to the second, or less than it, the third can be proved to be equal to the fourth, or less than it. Therefore, if the first, &c. Q. E. D.

* See Notes..