1. Write down a few of the leading terms of the series, and place their indices over them, beginning the indices with a unit or 1. 2. Add together such indices, whose sum shall make up the entire index to the sum required. 3. Multiply the terms of the geometrical series belonging to those indices together, and the product will be the term sought. EXAMPLES. 1. If the first be 2, and the ratio 2; what is the 13ih term ? 1, 2, 3, 4, 5, indices. Then 5+5+3=13. 2, 4, 8, 16, 32, leading terms. 32 x 32 x 8=8192 Ans. 2. A draper sold 20 yards of superfine cloth, the first yard for 3d., the second for 9d., the third for 27d., &c. in triple proportion geometrical; what did the cloth come to at that rate ? The 20th, or last term, is 3486784401d. Then 3+3486784401-3 5230176600d. the sum of all 3-1 the terms (by Prob. I.) equal to £21792402, 10s. 3. A rich miser thought 20 guineas a price too much for 12 fine horses, but agreed to give 4 cts. for the first, 16 cts. for the second, and 64 cents for the third horse, and so on in quadruple or fourfold proportion to the last: what did they come to at that rate, and how much did they cost per head one with another? Ans. The 12 horses came to $223696, 20 cts., and the average price was $18641, 35 cts. per head. Thus, 5 1 2 3 4 5, &c. indices or arithmetical series 2 4 8 16 32, &c. geometrical series. and 4*8 = 32 the fifth term. Now, | When the first term of the series and the ratio are diffe rent, that is, when the first term is either greater or less than the ratio.* 1. Write down a few of the leading terms of the series, and begin the indices with a cipher: Thus, 0, 1, 2, 3, &c 2. Add together the most convenient indices to make an index less by 1 than the number expressing the place of the terms sought. 3. Multiply the terms of the geometrical series together belonging to those indices, and make the product a dividend 4. Raise the first term to a power whose index is one less than the number of the terms multiplied, and make the result a divisor. 5. Divide, and the quotient is the term sought. i EXAMPLES, 4. If the first of a geometrical series be 4, and the ratio 2, what is the 7th term ? 0, 1, 2, 3, Indices, 3+2+1=6, the index of the 7th term. =2916 the 7th term required. 16 Here the number of terms multiplied are three; therefore the first term raised to a power less than three, is the 2d power or square of 4=16 the divisor. * When the first term of the series and the ratio are different, the indices must begin with a cipher, and the sum of the indices made choice of must be one less than the number of terms given in the question : because 1 in the indices stands over the second term, and 2 in the indices over the third term, &c. and in this case, the product of any two terms, divided by the first is equal to that term beyond the first, signified by the sum of their indices. 0, 1234. Thus, {i; 3; 9; 2*, 81, &c. Geometrical series. Here 4+3=7 the index of the 8th term. 81 x 27=2187 the 8th term, or the 7th beyond the 1sti 5. A Goldsmith sold 1 lb. of gold, at 2 cts. for the first punoe, 8 cents for the second, 32 cents for the third, &c.in a quadruple proportion geometrically: what did the whole come to ? Ans. $111848, 10 cts. 6. What debt can be discharged in a year, by paying 1 farthing the first month, 10 farthings, or (20) the second and so on, each month in a tenfold proportion? Ans. £115740740 14s. 9d. 3 qrs. 7. A thrasher worked 20 days for a farmer, and received for the first days work fou barley-corns, for the second 12 barley corns, for the third 36 barley corns, and so on, in triple proportion geometrically. I demand what the 20 day's labour came to supposing a pint of barley to contain 7680 corns, and the whole quantity to be sold at 2s. 6d. per bushel ? Ans. £1773 7s. 6d. rejecting remainders. 8. A man bought a horse, and by agreement, was to give a farthing for the first nail, two for the second, four for the third, &c. There were four shoes, and eight nails in each shoe ; what did the horse come to at that rate ? Ans. £4473924 58. 3 d. 9. Suppose a certain body, put in motion, should move the length of 1 barley-corn the first second of time, one inch the second, and three inches the third second of time, and so continue to increase its motion in triple proportion geometrical; how many yards would the said body move in the term of half a minute. Ans. 953199685623 yds. 1 ft. 1 in. 16. which is no less than five hundred and forty-one millions of miles, POSITION. SINGLE POSITION RULE.-1. Take any number and perform the same operation Ś with it, as is described to be performed in the question. 2. Then say; as the result of the operation is to the given sum in the question : : so is the supposed number : to the true one required. The method of proof is by substituting the answer in the ques tion. EXAMPLES, SC 1. A schoolmaster being asked how many scholars he lad, said, If I had as many more as I now have, balf as many, one-third, and one fourth as many, I should then have 148; How many scholars had he? Suppose he had 12 As 37 : 148 : : 12 : 48 Ans. as many 12 48 24 16 12 Proof, 148 2. What number is that which being increased by }, }, and of itself, the sum will be 125? Ans. 60. 3. Divide 93 dollars between A, B and C, so that B's share may be half as much as A's, and C's share three times as much as B's. Ans. A's share $31, B's $15, and C's $46. 4. A, B and C, joined their stock and gained 360 dols. of which A took up a certain sum, B took 3 times as much as A, and C took up as much as A and B both; what share of the gain had each? Ans. A $40, B $140, and C $180. 5. Delivered to a banker a certain sum of money, to receive interest for the same at 61, per cent. per annum, simple interest, and at the end of twelve years received 7311, principal and interest together; what was the sum delivered to him at first? Ans. £425. 6. A vessel has 3 cocks, A, B and C; A can fill it in 1 hour, B in 2 hours, and C in 4 hours ; in what time will they all fill it together? Ans. 34 min. 171 scc. DOUBLE POSITION, TEACHES to resolve questions by making two suppo sitions of false numbers.* RULE. 1. Take any two convenient numbers, and proceed with each according to the conditions of the question. 2. Find how much the results are different from the results in the question. 3. Multiply the first position by the last error, and the last position by the first error. 4. If the errors are alike, divide the difference of the products by the difference of the errors, and the quotient will be the answer. 5. If the errors are unlike, divide the sum of the products by the sum of the errors, and the quotient will be the answer. Note.—The errors are said to be alike when they are both too great, or both too small; and unlike, when one is too great, and the other too small. EXAMPLES. 1. A purse of 100 dollars is to be divided among 4 men, A, B, C and D, so that B may have four dollars more than A, and C 8 dollars more than B, and D twice as many as C; what is each one's share of the money? 1st. Suppose A6 2d. Suppose A 8 B 10 B 12 C 18 C20 D 36 D 40 * Those questions in which the results are not proportional to their posis tions, belong to this rule; such as those in which the number sought is in creased or diminished by some given number, which is no known part of the number required. |