23
ADDING ALGEBRAIC FRACTIONS
Different denominators -- The LCM
THERE IS ONE RULE for adding or subtracting fractions: The denominators must be the same.
| a c |
+ | b c |
= | a + b c |
Add the numerators, and place their sum
over the common denominator.
| Example 1. | 6x + 3 5 |
+ | 4x − 1 5 |
= | 10x + 2 5 |
The denominators are the same. Add the numerators as like terms.
| Example 2. | 6x + 3 5 |
− | 4x − 1 5 |
To subtract, change the signs of the subtrahend, and add.
| 6x + 3 5 |
− | 4x − 1 5 |
= | 6x + 3 − 4x + 1 5 |
= | 2x + 4 5 |
Problem 1.
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Do the problem yourself first!
| a) | x 3 |
+ | y 3 |
= | x + y 3 |
b) | 5 x |
− | 2 x |
= | 3 x |
| c) | x x − 1 |
+ | x + 1 x − 1 |
= | 2x + 1 x − 1 |
d) | 3x − 4 x − 4 |
+ | x − 5 x − 4 |
= | 4x − 9 x − 4 |
| e) | 6x + 1 x − 3 |
− | 4x + 5 x − 3 |
= | 6x + 1 − 4x − 5 x − 3 |
= | 2x − 4 x − 3 |
| f) | 2x − 3 x − 2 |
− | x − 4 x − 2 |
= | 2x − 3 − x + 4 x − 2 |
= | x + 1 x − 2 |
Different denominators -- The LCM
To add fractions with different denominators, we must first learn about the Lowest Common Multiple.
The Lowest Common Multiple (LCM) of a series of terms
is the smallest product that contains every factor from every term.
For example, here is a series of three terms:
pq pr ps
Let us construct their LCM factor by factor.
To begin, it will have the factors of the first term:
LCM = pq
Moving on to the second term, the LCM must have those factors, pr. But it already has the factor p. Therefore, we need only add the factor r:
LCM = pqr
Finally, moving on to the last term, the LCM must contain the factors ps. But again it has the factor p, and so we need only add the factor s:
LCM = pqrs.
This product is the Lowest Common Multiple of those three terms. It contains the factors pq, pr, ps. And it is is the smallest such product.
Example 3. Construct the LCM of these three terms: x, x², x3.
Solution. The LCM must have the factor x.
LCM = x
But it also must have the factors of x² -- which are x· x. Therefore, we need only add one more factor of x :
LCM = x²
Finally, the LCM must have the factors of x3, which are x· x· x. Therefore,
LCM = x3
x3 is the smallest product that contains x, x², and x3.
Problem 2. Construct the LCM of each series of terms.
| a) | ab, bc, cd abcd | b) | pqr, qrs, rst pqrst | |
| c) | a, a², a3, a4 a4 | d) | a²b, ab² a²b² | |
e) ab, cd abcd
We will now see what this has to do with adding fractions.
| Example 4. Add: | 3 ab |
+ | 4 bc |
+ | 5 cd |
Solution. To add fractions, the denominators must be the same. Therefore, as a common denominator choose the LCM of the original denominators. Choose abcd. Then, convert each fraction to an equivalent fraction with denominator abcd.
It is necessary to write the common denominator only once:
| 3 ab |
+ | 4 bc |
+ | 5 cd |
= | 3cd + 4ad + 5ab abcd |
| To change | 3 ab |
into an equivalent fraction with denominator abcd, |
we must multiply ab by the factors it is missing, namely, cd. Therefore, we must also multiply 3 by cd. That accounts for the first term in the numerator.
| To change | 4 bc |
into an equivalent fraction with denominator abcd, |
we must multiply bc by the factors it is missing, namely, ad. Therefore, we must also multiply 4 by ad. That accounts for the second term in the numerator.
| To change | 5 cd |
into an equivalent fraction with denominator abcd, |
we must multiply cd by the factors it is missing, namely, ab. Therefore, we must also multiply 4 by ab. That accounts for the last term in the numerator.
That is how to add fractions with different denominators.
Problem 3. Add.
| a) | 5 ab |
+ | 6 ac |
= | 5c + 6b abc |
| b) | 2 pq |
+ | 3 qr |
+ | 4 rs |
= | 2rs + 3ps + 4pq pqrs |
| c) | 7 ab |
+ | 8 bc |
+ | 9 abc |
= | 7c + 8a + 9 abc |
| d) | 1 a |
+ | 2 a² |
+ | 3 a3 |
= | a² + 2a + 3 a3 |
| e) | 3 a²b |
+ | 4 ab² |
= | 3b + 4a a²b² |
| f) | 5 ab |
+ | 6 cd |
= | 5cd + 6ab abcd |
| g) | _2_ x(x + 2) |
+ | __3__ (x + 2)(x − 3) |
= | 2(x − 3) + 3x x(x + 2)(x − 3) |
| = | _ 2x − 6 + 3x_ x(x + 2)(x − 3) |
||||
| = | _5x − 6_ x(x + 2)(x − 3) |
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Example 5. Denominators with no common factors.
| a m |
+ | b n |
When the denominators have no common factors, their LCM is simply their product, mn.
| a m |
+ | b n |
= | an + bm mn |
The numerator then appears as the result of "cross-multiplying" :
an + bm
This technique is appropriate, however, only when the denominators have no common factors.
| Example 6. | 2 x − 1 |
− | 1 x |
Solution. These denominators have no common factors -- x is not a factor of x − 1. It is a term. Therefore, the LCM of denominators is their product.
| 2 x − 1 |
− | 1 x |
= | 2x − (x − 1) (x − 1)x |
= | 2x − x + 1 (x − 1)x |
= | _x + 1_ (x − 1)x |
Note: The entire x − 1 is being subtracted. Therefore, we write it in parentheses -- and its signs change.
Problem 4.
| a) | x a |
+ | y b |
= | xb + ya ab |
b) | x 5 |
+ | 3x 2 |
= | 2x + 15x 10 |
= | 17x 10 |
| c) | 6 x − 1 |
+ | 3 x + 1 |
= | 6(x + 1) + 3(x − 1) (x + 1)(x − 1) |
| = | 6x + 6 + 3x − 3 (x + 1)(x − 1) |
||||
| = | _9x + 3_ (x + 1)(x − 1) |
||||
| d) | 6 x − 1 |
− | 3 x + 1 |
= | 6(x + 1) − 3(x − 1) (x + 1)(x − 1) |
| = | 6x + 6 − 3x + 3 (x + 1)(x − 1) |
||||
| = | _3x + 9_ (x + 1)(x − 1) |
||||
| e) | 3 x − 3 |
− | 2 x |
= | 3x − 2(x − 3) (x − 3)x |
| = | 3x − 2x + 6 (x − 3)x |
||||
| = | x + 6 (x − 3)x |
||||
| f) | 3 x − 3 |
− | 1 x |
= | 3x − (x − 3) (x − 3)x |
| = | 3x − x + 3 (x − 3)x |
||||
| = | 2x + 3 (x − 3)x |
||||
| g) | 1 x |
+ | 2 y |
+ | 3 z |
= | yz + 2xz + 3xy xyz |
| Example 7. Add: a + | b c |
Solution. We must express a with denominator c.
| a | = | ac c |
(Lesson 20) |
Therefore,
| a + | b c |
= | ac + b c |
Problem 5.
| a) | p q |
+ r | = | p + qr q |
b) | 1 x |
− 1 | = | 1 − x x |
| c) x − | 1 x |
= | x² − 1 x |
d) 1 − | 1 x² |
= | x² − 1 x² |
| e) 1 − | 1 x + 1 |
= | x + 1 − 1 x + 1 |
= | x x + 1 |
| f) 3 + | 2 x + 1 |
= | 3x + 3 + 2 x + 1 |
= | 3x + 5 x + 1 |
| Problem 6. Write the reciprocal of | 1 2 |
+ | 1 3 |
. |
| [Hint: Only a single fraction | a b |
has a reciprocal; it is | b a |
.] |
| 1 2 |
+ | 1 3 |
= | 3 + 2 6 |
= | 5 6 |
| Therefore, the reciprocal is | 6 5 |
. |
Next Lesson: Equations with fractions
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