Book V, is to A, so is D to C. Q. E. D. If, then, four magnitudes, &c. PROP. C. THEOR. See N. IF the first be the same multiple of the second, or the same part of it, that the third is of the fourth; the first is to the second as the third is to the fourth. Let the first A be the same multiple of B the second, that C the third is of the fourth D: A is to B as C is to D. Take of A and C any equimultiples whatever E and F; and of B and D any equimultiples whatever G and H: then because A is the same multiple of B that C is of D; and that E is the same multiple of A that F is of C; E is the same multiple of B that a 3. 5. F is of Da; therefore E and F are the same and G, H any equimultiples whatever of B, b 5.def.5. D. Therefore A is to B as C is to Db. Book V. PROP. D. THEOR. IF the first be to the second as to the third to the See N. fourth, and if the first be a multiple, or part of the second; the third is the same multiple, or the same part of the fourth. W : B Let A be to B as C is to D; and first let A be a multiple of B; C is the same multiple of D. Take E equal to A, and whatever multiple A or E is of B, make F the same multiple of D: then, because A is to B as C is to D; and of B the second and D the four h equimultiples have been taken E and F; A is to É as C to Fa: but A is equal to a Cor.4.5. E, therefore C is equal to Fb: and F is b A.5. the same multiple of D that A is of B. А Wherefore C is the same multiple of D that A is of B. E F Next, Let the first A be a part of the se See the cond B; C the third is the same part of the figure at fourth D. the foot Because A is to B as C is to D; then, of the inversely, B is c to A as D to C: but A is ing page: a part of B, therefore B is a multiple of A; c B. 5. and, by the preceding case, D is the same multiple of C, that is, C is the same part of D, that A is of B. Therefore, if the first, &c. Q. E. D. preced PROP. VII. THEOR. EQUAL magnitudes have the same ratio to the same magnitude; and the same has the same ratio to equal magnitudes. Let A and B be equal magnitudes, and C any other. A and B have each of them the same ratio to C, and has the same ratio to each of the magnitudes A and B. Take of A and B any equimultiples whatever D and E, and : Book V. of C any multiple whatever F: then, because D is the same multiple of A that E is of B, and that A is a 1.Ax.5. equal to B; D is a equal to E: therefore, if D be greater than F, E is greater than F; are any equimultiples of A, B, and F is any b 5. def.5. multiple of C. Therefore b, as A is to C, so is B to C. Likewise C has the same ratio to A, that D A E B C F See N. OF unequal magnitudes, the greater has a greater ratio to the same than the less has; and the same magnitude has a greater ratio to the less, than it has to the greater. Fig. 1. Let AB, BC be unequal magnitudes, of which AB is the greater, and let D be any magnitude whatever: AB has a greater ratio to D than BC to D: and D has a greater ra E tio to BC than unto AB. If the magnitude which is not the greater of the two AC, CB, be not less . A than D, take EF, FG, the doubles of F AC, CB, as in Fig. 1. But, if that which is not the greater of the two AC, CB be less than D (as in Fig. 2. and 3.) this magnitude can be multiplied, so as to G B become greater than D, whether it be AC, or CB. Let it be multiplied until L K HD it become greater than D, and let the other be multiplied as often; and let EF be the multiple thus taken of AC, and FG the same multiple of CB: therefore EF and FG are each of them greater than Fig. 3. D: and in every one of the cases, take H the double of D, K Book V. its triple, and so on, till the multiple of D be that which first becomes greater than FG: let L be that multiple of D which is first greater than FG, and K the multiple of D which is next less than L. Then, because L is the multiple of D, which is the first that becomes greater than FG, the next preceding multiple K is not greater than FG; that is, FG is not less than K: and since EF is the same multiple of AC, that FG is of CB; FG is the same multiple of CB, that EG is of AB a; wherefore EG and a 1. 5. FG are equimultiples of AB and CB: and it was shown, that FG was not less than Fig. 2. K, and, by the con E struction, EF is great E er than D; therefore F the whole EG is greater than K and D together : but K, together A A with D, is equal to L; therefore EG is greater than L; but FG is not greater than L; F and EG, FG are equi Cmultiples of AB, BC, G B G B and L is a multiple of D; therefore b AB has b 7. def. L Κ Η D L KD to D a greater ratio 3. than BC has to D. Also, D has to BC a greater ratio than it has to AB: for, having made the same construction, it may be shown, in like manner, that L is greater than FG, but that it is not greater than EG: and L is a multiple of D; and FG, EG are equimultiples of CB, AB: therefore D has to CB a greater ratio b than it has to AB. Wherefore, of unequal magnitudes, &c. Q. E. D. Book V. PROP. IX. THEOR. See N. MAGNITUDES which have the same ratio to the same magnitude are equal to one another; and those to which the same magnitude has the same ratio are equal to one another. Let A, B have each of them the same ratio to C: A is equal to B: for, if they are not equal, one of them is greater than the other; let A be the greater; then, by what was shown in the preceding proposition, there are some equimultiples of A and B, and some multiple of C such, that the multiple of A is greater than the multiple of C, but the multiple of B is not greater than that of C. Let such multiples be taken, and let D, E be the equimultiples of A, B, and F the multiple of C, so that D may be greater than F, and E not greater than F: but, because A is to C as B is to C, and of A, B are taken equimultiples D, E, and of C is taken a multiple F; and that D is greater than F; a 5. def. E shall also be greater than Fa; but E is D A Next, Let C have the same ratio to each E cl |