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124. RULE III. To divide a fraction by an integer : divide the numerator, if it be divisible, by the integer; or, if the numerator be not divisible, multiply the denominator by that integer.
The rule may be proved as follows:
represents a equal parts, b of which make up the unit.
The number of parts taken in the first fraction is c times the number taken in the second. Therefore the second fraction is the quotient of the first fraction divided by c; that is,
125. To find the value of 7 of a By definition, [Art. 119.]
of a represents a equal parts, 6 of which make up
126. RULE IV. To multiply together two or more fractions : multiply the numerators for a new numerator, and the denominator for a new denominator.
The definition of multiplication, in the strict sense of the word, supposes that a quantity is to be added to itself a certain number of times. But when the multiplier is a fraction this definition ceases to be intelligible; the operation can therefore be only understood in some extended sense. To find a meaning 7 d' Х
times we may consider that to obtain
we must perform on
an operation similar to that we perform on the unit to obtain à In this case we take '
of 1; hence in the former we take of
d 7 Thus
127. RULE V. To divide one fraction by another: invert the divisor, and proceed as in multiplication.
Since division is the inverse of multiplication, we may define the quotient 2, when ő is divided by a, to be such that
2a2 + 3a 4a– 6a a (2a+3) 2a (2a – 3)
2a - 3
12a by cancelling those factors which are common to both numerator and denominator.
2.c + a
6x2 - ax - 2a2 Example 2. Simplify
3ax + 2a?"
6x2 – AX –
- 2a The expression=
2- a Х
9x2 – 4a
3ax + 2a2 Х
2x + a
(3x – 2a) (2.c + a)
EXAMPLES XV. c.
14.2 - 7x 2x - 1 1. •
2. 12.03 +24.c.222 +2.x®
x2 - 4a? 2a
X – 2
6. x2 - 4 4x - 3a'
x2 + 5x + 6 22 - 2x - 3 7.
2.02 + 13x + 15 2.x2 +11x +5
4.25 - 1 22 - 141 – 15 22 - 12.x – 45 11.
32 - 4x – 45 22 - 6x – 27
Thus in each case we divide the unit into bd equal parts, and we take first ad of these parts, and then bc of them; that is, we take ad+bc of the bd parts of the unit; and this is expressed
ad+bc by the fraction
ad+bc ē td bd
ad - bc Similarly,
d bd 129. Here the fractions have been both expressed with a common denominator bd. But if b and d have a common factor, the product bd is not the lowest common denominator, and the
ad + bc fraction will not be in its lowest terms. To avoid work
bd ing with fractions which are not in their lowest terms, some modification of the above will be necessary. In practice it will be found advisable to take the lowest common denominator, which is the lowest common multiple of the denominators of the given fractions.
RULE I. To reduce fractions to their lowest common denominator: find the L.C.M. of the given denominators, and take it for the common denominator ; divide it by the denominator of the first fraction, and multiply the numerator of this fraction by the quotient so obtained ; and do the same with all the other given fractions. Example. Express with lowest common denominator
3x (x2 – a)
We must therefore multiply the numerators by 3x (x + a) and 2a respectively. Hence the equivalent fractions are 15x2 (x + a)
and 6ax (x – a) (x + a) 6ax (0 - a) (++a) 130. We may now enunciate the rule for the addition or subtraction of fractions.
RULE II. To add or subtract fructions: reduce them to the lowest common denominator; add or subtract the numerators, and retain the common denominator.
2.x to a
5x – 4a Example 1. Find the value of
9a The lowest common denominator is 9a. Therefore the expression =
3 (2x + a) + 5x – 4a
11x - a
2y 3y - a 3x – 2a Example 2. Find the value of
xy ay The lowest common denominator is axy. Thus the expression=
a (x – 2y) + x (3y – a) – y (3.2 – 2a)
=0, since the terms in the numerator destroy each other.
NOTE. To ensure accuracy the beginner is recommended to use brackets as in the first line of work above.