Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

4 3
8

of

.

4 3 56.

out affecting the result. Therefore, to take a fraction of a fraction, that is, to multiply one fraction by another, multiply the denominators together for a new. denominator, and the numerators for

a new numerator. X If 7 dollars will buy 5 bushels of rye, how much will 1 dollar buy? How much will 15 dollars buy ?

1 dollar will buy of 5ş bushels. In order to find 7, of it, 5 must be changed to eighths. 53 4 = 1 dollar will buy ** of a bushel. 15 dollars will buy 15 times as much. 15 times = 6* = 113.. Ans. U bushels.

If 13 bols. of beef cost 952 dollars, what will 25 bbls. cost?

1 bbl. will cost is of 95 dollars, and 25 bbls. will cost of it. To and this, it is best to multiply first by 25, and then divide by 13. For of 95 is the same as its of 25 times. 95.

Operation. 957 x 25 = 2396]. 23967 (13

13

[merged small][ocr errors][merged small][merged small]

4= 'Ans. 1848 dolls. In this example I divide" 23967 by 13. I obtain a quotient 184, and a remainder 4}, which is equal to 3. Then divided by 13, gives 101, which I annex 10 the quotient, and the division is completed.

The examples hitherto employed to illustrate the division of fractions, have been such as to require the division of the fractions into parts. It has been shown (Art. XVI.) that the division of whole numbers is performed in the same manner, whether it be required to divide the number into parts, or to find how many

times one number is contained in another. It will now be shown that the same is true with regard to fractions.

It 3 dollars a barrel, how many barrels of cider may be bought for S dollars ?

The numbers must be reduced to fifths, for the same reason that they must be reduced to pence, if one of the numbers were given in shillings and pence.

3= 14, and 8 = 43. As many times as are contained in 13, that is, as many times as 15 are contained in 43, so many barrels may be bought.

Expressing the division = 21. Ans. 24 barrels. This result agrees with the manner explained above. For 8 was reduced to fifths, and the denominator 15 was formed by multiplying the denominator 5 by the divisor 3.

How many times is 2 contained in ?

2 = 24: 14 is contained in 5, of one time. The same result may be produced by the other ‘method.

XVIII. We have seen that a fraction may be divided by multiplying its denominator, because the parts are made smaller. On the contrary, a fraction may be multiplied by dividing the denominator, because the parts will be made larger. If the denominator be divided by 2, for instance, the denominator being rendered only half as large, the unit will be divided into only one half as many parts, consequently, the parts will be twice as large as before. If the denominator be divided by 3, the unit will be divided into only one third as many parts, consequently the parts will be three times as large as before, and if the same number of these parts be taken, the value of the fraction will be three times as great, and so on.

If 1 lb. of sugar cost of a dollar, what will 4 lbs. cost?

If the denominator 8 be divided by 4, the fraction becomess; that is, the dollar, instead of being divided into S paris, is divided into only 2 parts. It is evident that halves are 4 times as large as eighths, because if each half be divided into 4 parts, the parts will be eighths. Ans. doll.

If it be done by multiplying the numerator, the answer is , which is the same as i, for i = 1, and į of 5 = .

21

2's, and

If i lb. of figs cost of a dollar, what will 7 lbs. cost?

Dividing the denominator by 7, the fraction becomes . Now it is evident that fourtiis are 7 times as large as twenty-eighths, because if fourths be divided into 7 parts, the parts will be twenty-eights. Ans dolls.

Or multiplying the numerator, 7 times z's is it. But 1

, so that the answers are the same. Therefore, to multiply a fraction, divide the denominator, when it can be done without a remainder.

Two ways have now been found to multiply fractions, and two ways to divide them.

To multiply a fraction The numerator, Art. 15.
To divide a fraction 5* The denominator, Art. 17.
To divide a fraction
To multiply a fractions The denominator, Art. 1s.

XIX. We observed a remarkable circumstance in the last article, viz. that į = and j = it This will be found very important in what follows.

A man having a cask of wine, sold į of it at one time, and į of it at another; how much had he left?

and cannot be added together, because the parts are of different values. Their sum must be more than f, and less than or 1. If we have dollars and crowns to add together, we reduce them both to pence. Let us see if these fractions cannot be reduced both to the same denomination. Now === , &c.

&c. And 27 / == , &c. It appears, therefore, that they may both be changed to sixths. į = Å and

î, which added together make. He had sold and had a left.

A man sold of a barrel of flour at one time, and 4 at another; how much did he sell in the whole?

Fifths and sevenths are different parts, but if a thing be first divided into 5 equal parts, and then those parts each into 7 equal parts, the parts will be thirty-fifths. Also if the thing be divided first into 7 equal parts, and then those parts each into 5 equal parts, the parts will be thirty-fifths. Therefore, the parts will be alike. But in dividing them thus, will make }, and will make by 28.

s, and the two added together make }, that is, 19. Ans. I, barrel.

When the denominators of two or more fractions are alike, they are said to have a common denominator, And the process by which they are made alike, is called reducing them to a common denominator.

In order to reduce pounds to shillings, we multiply by 20, and to reduce guineas to shillings, we multiply

In like manner to reduce two or more fractions to a common denominator, it is necessary to find what denomination they may be reduced to, and what number the parts of each must be multiplied by, to reduce them to that denomination.

If the denominator of a fraction be multiplied by 2, it is the same as if each of the parts were divided into 2 equal parts, therefore it will take 2 parts of the latter kind to make 1 of the former. If the denominator be multiplied by 3, it is the same as if the parts were divided each into 3 equal parts, and it will take 3 parts of the latter kind, to make 1 of the former. Indeed, whatever number the denominator be multiplied by, it is the same as if the parts were each divided into so many equal parts, and it will take so many parts of the latter kind to make 1 of the former. Therefore, to find what the parts must be multiplied by, it is necessary to find what the denominator must be inultiplied by to produce the denominator required.

The common denominator tlien, (which must be found first) must be a number of which the denominators of all thie fractions to be reduced, are factors. We shall always find such a number, by multiplying the denominators together. Hence if there are only tivo fractions, the denominators being multiplied together for the common denominator, the parts of one fraction must be multiplied by the denominator of the other. If there be more than iwo fractions, since by multiplying all the denominators together, the denominator of each will be multiplied by all the others, the parts in each fraction, that is, the numerators, must be multiplied by the denominators of the other fractions.

In the above example to reduce and to a common denominator, 7 times 5 are 35; 7 is the number by which the first denominator 5 must be multiplied to produce 35, and consequently the number by which the numerator 3 must be multiplied. 5 is the number, by which 7, the second denominator, must be multiplied to produce 35, and consequently the number by which the numerator 4 must be multiplied.

N. B. It appears from the above reasoning, that if both the numerator and denominator of any fraction be multiplied by the same number, the value of the fraction will remain the same. It will follow also from this, that if both numerator and denominator can be divided by the same number, without a remainder, the value of the fraction will not be altered. In fact, if the numerator be divided by any number, as 3 for example, it is taking

of the number of parts; then if the denominator be divided by 3, these parts will be made 3 times as large as before, consequently the value will be the same as at first. This enables us frequently, when a fraction is expressed with large numbers, to reduce it, and express it with much smaller numbers, which often saves a great deal of labour in the operations.

Take for example 1. Dividing the numerator by 5, we take } of the parts, then dividing the denominator by 5, the parts are made 5 times as large, and the fraction becomes , the same value as is. This is called reducing fractions to lower terms. Hence

To reduce a fraction to lower terms, divide both the numerator and denominator by any number that will divide them both without a remainder.

Note. This gives rise to a question, how to find the divisors of numbers. These may frequently be found by trial. The question will be examined hereafter.

A man bought four pieces of cloth, the first contained 238 yards; the second 28282; the third 37 ; and the fourth 17). How many yards in the whole

The fractional parts of these numbers cannot be added together until they are reduced to a common de

« ΠροηγούμενηΣυνέχεια »