Book V. PROP. III. THEOR. If 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 F magnitudes EK, KF, each equal to A, and GH into GL, LH, H 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 KŤ 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, there E A B G C D fore, 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 a of the second B, which GH a 2. $. the third, together with the sixth, is of the fourth D. If, there. fore, the first, &c. Q. E. D. Book V. PROP. IV. THEOR. See N. IF the first of four magnitudes has the same ratio to the second which the third hath 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.' 6 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 a 3. 5. is of Ca: for the same reason M K E А B G M is the same multiple of B, that N. is of D: and because as A is to L F C DH N b Hyp. B, so is C to Db, and of A and C have been taken certain equi- N; and if equal, equal; if less, 65. def.5. less. And K, L are any equi multiples whatever of E, F; and Q. E. D. which the third has to the fourth, then also any equimultiples whatever of the first and third have the same ratio to the se. Book V. cond 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 before, 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, lessc: and K c5 def.5 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 the other case is demonstrated. PROP. V. THEOR. IF one magnitude be the same multiple of another, See N. 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 G 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 A the whole AB is of the whole CD. Take AG the same multiple of FD, that AE is of CF: therefore AE is a the same mul с a 1. 5. tiple 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 mul E tiple of CD that AB is of CD; wherefore EG is equal to ABb. 'Take from them the common b 1. Ax.5. magnitude AE; the remainder AG is equal to the remainder F.B. Wherefore, since AE is B 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 Book V. 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. See N. IF 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. First, Let GB be equal to E; HD is A K CD is of F; wherefore KH is equal to al. Ax. 5. CD a : take away the common magni B D E F But, let GB be a multiple of E; then A K AB is the same multiple of E that KH b 2. 5. is of Fb: but AB is the same multiple of H- B D E F 1 Book V. PROP. A. THEOR. IF the first of four magnitudes has to the second the See N. 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. PROP. B. THEOR. IF four magnitudes are proportionals, they are See N. proportionals also when taken inversely. If the magnitude A be to B as C is to D, then also inversely B is to A as D to C. Take of B and D any equimultiples whatever E and F; and of A and C any equimultiples whatever G and H. First, Let E be greater than G, then G is less than E; and, because A is to B as C is to D, and of A and C, the first and third, G and H are equimultiples ; and of B and D, the second and fourth, E and F are equimultiples; and that G is less than E, H is also G A E a less than F; that is, F is greater than H; a 5. def. if therefore E be greater than G, F is great- H с D F 5. er than H : in like manner, if E be equal to G, F may be shown to be equal to H; and if less, less; and E, F are any equimultiples whatever of B and D, and G, H any whatever of A and C; therefore, as B R |