Concept

Identifying Missing Prime Factors for Equivalent Fractions

When the LCD of two fractions has been determined through prime factorization, a shortcut called the missing factors method reveals the exact multiplier needed for each fraction. Compare each denominator's prime factorization column-by-column against the LCD's factorization; any prime present in the LCD but absent from a particular denominator is a "missing factor" for that denominator. The product of all missing factors is the number by which both the numerator and denominator of that fraction must be multiplied.

For example, if the LCD is 2233=362 \cdot 2 \cdot 3 \cdot 3 = 36:

  • The denominator 12=22312 = 2 \cdot 2 \cdot 3 is missing one factor of 33, so multiply by 33.
  • The denominator 18=23318 = 2 \cdot 3 \cdot 3 is missing one factor of 22, so multiply by 22.

This technique eliminates guesswork when building equivalent fractions with the LCD and works whenever the LCD was found using the prime factors method.

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Updated 2026-04-21

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