Activity (Process)

How to Add or Subtract Rational Expressions

To add or subtract rational expressions — whether or not they share the same denominator — follow a three-step procedure:

Step 1. Determine whether the expressions have a common denominator.

  • If yes, proceed directly to Step 2.
  • If no, rewrite each rational expression so that it has the LCD as its denominator:
  • Find the LCD using the four-step LCD procedure (factor each denominator completely, list and match factors vertically, bring down one factor per column, and multiply).
  • Multiply the numerator and denominator of each rational expression by the factor(s) it is missing from the LCD, producing equivalent rational expressions that all share the LCD.

Step 2. Add or subtract the rational expressions by combining the numerators over the common denominator. The process used to subtract rational expressions with different denominators is the same as for addition, but requires extra care with signs when subtracting the numerators (ensuring the negative is distributed to all terms).

Step 3. Simplify the result, if possible, by factoring the numerator and denominator completely and dividing out any common polynomial factors.

This procedure extends the numerical fraction strategy for addition and subtraction to rational expressions. The key difference is that finding the LCD and simplifying both involve polynomial factoring rather than prime factoring, and the LCD is typically left in factored form.

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Updated 2026-04-30

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