Adding and Subtracting Rational Expressions

Method

Finding the Least Common Multiple of Polynomials

To find the least common multiple (LCM) of two or more polynomials, the polynomials must be factored completely. The LCM is the product of the factors with the highest power that appear in any of the polynomials. To show an example, the LCM of the following polynomials will be found. Polynomial I: & 12x^2y+48xy+48y Polynomial II: & 3x^3y-18x^2y-48xy The procedure of finding the LCM of the polynomials involves three steps.

1
Factor Each Polynomial and Write Numerical Factors as Product of Primes
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Each polynomial will be factored. Start by factoring the first polynomial. Note that 12y is used in all of its terms. Therefore, it will be factored out.

12x^2y+48xy+48y
12y * x^2 +12y * 4x + 12y * 4
12y (x^2+4x+4)
12y (x+2)^2

Then the numerical factor can be written as a product of 2^2 and 3. Polynomial I [0.5em] 12x^2y+48xy+48y ⇕ 2^2 * 3 * y (x+2)^2 Now the other polynomial will be factored. Since the factor 3xy can be seen in each of its terms, start by factoring it out.

3x^3y-18x^2y-48xy
3xy * x^2-3xy * 6x-3xy * 16
3xy(x^2-6x-16)
Factor (x^2-6x-16)
3xy(x^2-8x+2x-16)
3xy (x(x-8)+2x-16 )
3xy(x(x-8)+2(x-8) )
3xy(x+2)(x-8)

2
Identify the Factors With the Highest Power
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Both polynomials are now written in factored form. Now, write all the missing related factors to identify the factor with the highest power.

Standard Form Factored Form All Related Factors
Polynomail I 12x^2y+48xy+48y 2^2 * 3 * y * (x+2)^2 2^2 * 3^1 * x^0 * y^() 1 * (x+2)^2 * (x-8)^1
Polynomail II 3x^3y-18x^2y-48xy 3 * x * y * (x+2) * (x-8) 2^0 * 3^1 * x^1 * y^() 1 * (x+2)^1 * (x-8)^1

The highest power of each prime factor can be listed as follows. 2^2, 3^1, x^1, y^1, (x+2)^2, and (x-8)^1

3
Multiply the Highest Power of the Factors
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The LCM of the polynomials is the product of the factors listed in the previous step.

2^2 * 3^1 * x^1 * y^1 * (x+2)^2 * (x-8)^1
4* 3* x * y* (x+2)^2* (x-8)
12xy(x+2)^2(x-8)

As shown, the least common multiple of the given polynomials is another polynomial.

Exercises
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