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How to Find the LCD of Rational Expressions

To find the least common denominator of two or more rational expressions, apply a four-step procedure that parallels the prime factors method used for numerical fractions — but uses polynomial factors instead of prime numbers, and the LCD is left in factored form rather than multiplied out:

Step 1. Factor each denominator completely.

Step 2. List the factors of each expression, matching common factors vertically when possible.

Step 3. Bring down one factor from each column — shared factors are counted only once, while all distinct factors are included.

Step 4. Multiply the factors. The product (left in factored form) is the LCD.

This procedure mirrors the prime factors method for finding the LCM of numbers: factoring into primes corresponds to factoring polynomials, and lining up common primes corresponds to lining up common binomial or polynomial factors. The key difference is that for rational expressions, the LCD is typically left as a product of polynomial factors rather than being multiplied out into a single expanded polynomial.

As with all rational expressions, values of the variable that would make any denominator equal zero must be excluded.

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Updated 2026-05-01

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