# Adding and Subtracting Rational Expressions

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Adding and Subtracting Rational Expressions
Goal Determine the LCM of polynomials Goal Add and Subtract Rational Expressions

What is the Least Common Multiple?
Fractions require you to find the Least Common Multiple (LCM) in order to add and subtract them!                                        Least Common Multiple (LCM) - smallest number or polynomial into which each of the numbers or polynomials will divide evenly. The Least Common Denominator is the LCM of the denominators.

Find the LCM of each set of Polynomials
1) 12y2, 6x2 LCM = 12x2y2 2) 16ab3, 5a2b2, 20ac LCM = 80a2b3c 3) x2 – 2x, x2 - 4 LCM = x(x + 2)(x – 2) 4) x2 – x – 20, x2 + 6x + 8 LCM = (x + 4) (x – 5) (x + 2)

Adding Fractions - A Review
LCD is 12. Find equivalent fractions using the LCD. Collect the numerators, keeping the LCD.

Remember: When adding or subtracting fractions, you need a common denominator!
When Multiplying or Dividing Fractions, you don’t need a common Denominator

Steps for Adding and Subtracting Rational Expressions:
1. Factor If possible, Cancel common factors directly over each other. Get a common denominator List out factors Multiply by 1’s Add the top (combine like terms) Re-factor and simplify if possible 4. Reduce, if possible. 5. Leave the denominators in factored form.

Addition and Subtraction Is the denominator the same??
Example: Simplify Find the LCD: 6x Now, rewrite the expression using the LCD of 6x Simplify... Add the fractions... = 19 6x

Simplify: Example 1 6(3mn2) + 8(5n) - 7(15m)

Adding and Subtracting with polynomials as denominators
Simplify: Find the LCD: (x + 2)(x – 2) Rewrite the expression using the LCD of (x + 2)(x – 2) Simplify... – 5x – 22 (x + 2)(x – 2)

Adding and Subtracting with Binomial Denominators
(x + 1) + 3 (x + 3) 2

Simplify: Example 6 ** Needs a common denominator 1st! Sometimes it helps to factor the denominators to make it easier to find your LCD. LCD: 3x3(2x+1)

Simplify: Example 7 LCD: (x+3)2(x-3)

Simplify: 3x (x - 1) - 2x (x + 2) (x + 3)(x + 2) (x + 3)(x - 1) LCD
Example 9 3x (x - 1) - 2x (x + 2) (x + 3)(x + 2) (x + 3)(x - 1) LCD (x + 3)(x + 2)(x - 1) x ≠ -3, -2, 1

Simplify: 4x (x - 2) + 5x (x - 3) (x - 3)(x - 2) (x - 2)(x - 2) LCD
Example 10 4x (x - 2) + 5x (x - 3) (x - 3)(x - 2) (x - 2)(x - 2) LCD (x - 3)(x - 2)(x - 2) x ≠ 3, 2

Simplify: (x + 3) (x - 2) - (x - 4) (x + 1) Example 11 (x - 1)(x + 1)
LCD (x - 1)(x + 1)(x - 2) x ≠ 1, -1, 2