« ΠροηγούμενηΣυνέχεια »
rent, that is, when the first term is either greater or
1. Write down a few of the leading terms ef tiie scries, and begin the indices with a cypher: 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 term sought.
S. 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 wliose index is one less than the number of the terms multiplier, and make the resulta divisor.
5. Divide, and the quotient is the term sought.
4. If the first of a geometrical series be 4, and the rati* S, what is the 7th term ?
0, 1, 2, 3, Indices.
3+2+1=6, the index of the th term.
2916 the th term required
16 Here the number of terms multiplied are three; there. fore the first term raised to a power less than three, is the ad power or square
of 4=16 the divisor. *When the first tern of the series and the ratio are different, the indices inust begin with a cypher, and the sum of the indices made choice of uust be one less than the number of terms given in the question: because l in the indices stand's over the second terin, and 2 in the indices over the third term, &'c. and in this case, the product of any tuo terms, divided by the first, is equal to that term beyond the first, signified by the sum of their indices. Thus,
Ş0, 1, 2, 3, 4, 8c. Indices.
21, 3, 9, 27, 81, 8c. Geometrical series. Here 4+3=7 the index of the 8th term.
81X27=2187 the 8th term, or the 7th beyond the 1st
5. A Goldsmith sold 1 lb. of gold, at 2 cents for the first ounce, 3 cents for the second, 52 cents for the third, &c. in a quauruple proportion geometrically; what did the whole come to?
Ans. $111848, 10cks. 6. What debt can be discharged in a year, by paying 1 farthing the first month, 10 farthings, (or 24d.) the second, and so on, each month in a tenfold proportion?
Ans. £115740740 14s. 9d. 3qrs. 7. A thresher worked 20 days for a farmer, and receive ed for the first day's work four barley-corns, for the second 12 barley-corns, for the third 36 barley-corns, and so on in triple proportion geometrical. I demand what the 20 days’labor 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 shue ; what did the horse come to at that rate ?
Ans. £4473924 5s. 31d. 9. Suppose a certain body, put in motion, should move the length of one 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 ?
Ars. 953199685623 yds. ift. lin. 1b.c. which is no less tran five hundred and forty-one millions of miles.
POSITION. POSITION is a rule which, by false or supposed numbers, taken at pleasure, discovers the true ones required. It is divided into two parts, Single or Double.
SINGLE POSITION, Is when one number is required, the properties of which are given in the question.
RULE. 1. Take any number and perform the same ope with it, as is described to be performed in the quest
2. Then say; as the result of the operation : is given sum in the question : : so is the supposed nu to the true one required.
The method of proof is by substituting the ansı the question.
1. A schoolmaster being asked how many schol had, said, If I had as many more as I now have, h many, one-third and one-fourth as many, I should have 148; How many
scholars had he ? Suppose he had 12 As 37 : 148 : : 12 : 48 as many
48 as many 6
24 as many
16 as many 3
12 Result, 37
Proof, 148 2. What number is that which being increased by amd 3 of itself, the sum will be 125 ? Ansic
3. Divide 93 dollars between A, B and C, so tha share
may be half as much as A's, and C's share times as much as B's.
Ans. A's share $31, B's $15), and C's 84 4. A, B and C, joined their stock and gained 560 of which A took up a certain sum, B took 51 time much as A, and C took up as much as A and B what share of the gain had each :
Ans. A $40, B $140, and C 818 5. Delivered to a banker a certain sum of mone receive interest for the same at 6l.
cent. simple interest, and at the end of twelve years rece 7311. principal and interest together; what was the delivered him at first ?
6. A vessel has 3 cocks, A, B and C; A can fill 1 hour, B in 2 hours and C in 4 hours; in what time they all fill it together?
Ans. 34min. 17.se
DOUBLE POSITION, TEACHES to resolve questions by making two'suppe 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 pro ducts 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.
1. A purse of 100 dollars is to be divided among 4 men, A, B, C and D, so that B may have 4 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 A8
2d. error 20
* Those questions, in which the results are not propor. tional to their positions, belong to this rule ; such as those in which the number sought is increased or diminished by some given number, which is no known part of the number required.
The errors being alike, are both too small, there
10 120(12 A's part.
2. A, B, and C, built a house which cost 500 do of which A paid a certain sum; B paid 10 dollars than A, and C paid as much as A and B both ; how did each man pay?
Ans. A paid $120, B $150, and C $25 3. A man bequeathed 1001. to three of his friends, this manner : the first must have a certain portion, second must have twice as much as the first, wanting and the third must have three times as much as the i wanting 151.; I demand how much each man must ha
Ans. The first £20 10s, second 633, third £46
4. A laborer was hired 60 days upon this conditi that for every day he wrought he should receive 4s. for every day he was idle should forfeit 25., at the e ration of the time he received 71. 10s.; how many a did he work, and how maty was he idle ?
Ans. He wrought 45 days, and was idle 15 day 5. What number is that which being increased by , its , and 18 more, will be doubled ? Anis, TE
6. A man gave to his three sons all his estate in mon viz. to F half, wanting 501. to G one-third, and to H rest, which was 101. less than the share of G; I dema the sum given, and each man's part? Ans, the sui given was £560, whereof F had £1
G £120, and H £110.