Formula

Addition and Subtraction of Rational Expressions with a Common Denominator

The rules for adding and subtracting numerical fractions with a common denominator extend directly to rational expressions. If pp, qq, and rr are polynomials with q0q \neq 0, then:

pq+rq=p+rq\frac{p}{q} + \frac{r}{q} = \frac{p + r}{q}

pqrq=prq\frac{p}{q} - \frac{r}{q} = \frac{p - r}{q}

To add or subtract rational expressions that share the same denominator, combine the numerators — adding or subtracting them as polynomials — and write the result over the common denominator. Unlike numerical fraction addition, where the result is simplified by dividing out common numerical factors, rational expression results often require factoring the combined polynomial numerator and denominator completely so that common polynomial factors can be identified and canceled. The denominator polynomial remains unchanged throughout the operation; only the numerator polynomials are combined.

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Updated 2026-05-01

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