Greatest Common Factor

Arithmetic
definition

Also known as: GCF, GCD, greatest common divisor

Grade 6-8

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The greatest common factor (GCF) of two or more numbers is the largest positive integer that divides each of them evenly, with no remainder. Essential for reducing fractions to lowest terms: \frac{12}{18} = \frac{2}{3} because \gcd(12,18) = 6.

Definition

The greatest common factor (GCF) of two or more numbers is the largest positive integer that divides each of them evenly, with no remainder. It is also called the greatest common divisor (GCD).

💡 Intuition

The biggest 'piece' size that fits evenly into two numbers—like the largest tile that covers both a 12-unit and 18-unit floor.

🎯 Core Idea

GCF finds the largest common building block shared by numbers.

Example

GCF of 12 and 18: Factors of 12 (1,2,3,4,6,12) and 18 (1,2,3,6,9,18). GCF = 6.

Formula

\text{GCF}(a, b) \times \text{LCM}(a, b) = a \times b

Notation

\text{GCF}(a, b) or \gcd(a, b) denotes the greatest common factor of a and b

🌟 Why It Matters

Essential for reducing fractions to lowest terms: \frac{12}{18} = \frac{2}{3} because \gcd(12,18) = 6.

💭 Hint When Stuck

Write the prime factorization of both numbers, then circle the primes they share. Multiply the shared primes using the smaller exponent of each.

Formal View

\gcd(a, b) = \max\{d \in \mathbb{Z}^+ : d \mid a \text{ and } d \mid b\}. Via prime factorization: if a = \prod p_i^{\alpha_i} and b = \prod p_i^{\beta_i}, then \gcd(a,b) = \prod p_i^{\min(\alpha_i, \beta_i)}.

🚧 Common Stuck Point

Using prime factorization: GCF uses the smaller power of each common prime.

⚠️ Common Mistakes

  • Confusing GCF with LCM — GCF of 12 and 18 is 6 (largest common factor), while LCM is 36 (smallest common multiple)
  • Taking the larger power of each prime instead of the smaller — for 12 = 2^2 \times 3 and 18 = 2 \times 3^2, the GCF uses 2^1 and 3^1, giving 6, not 2^2 \times 3^2 = 36
  • Stopping at the first common factor found — finding that 2 divides both 12 and 18, but not checking for the greatest: 6 is the GCF, not 2

Frequently Asked Questions

What is Greatest Common Factor in Math?

The greatest common factor (GCF) of two or more numbers is the largest positive integer that divides each of them evenly, with no remainder. It is also called the greatest common divisor (GCD).

What is the Greatest Common Factor formula?

\text{GCF}(a, b) \times \text{LCM}(a, b) = a \times b

When do you use Greatest Common Factor?

Write the prime factorization of both numbers, then circle the primes they share. Multiply the shared primes using the smaller exponent of each.

How Greatest Common Factor Connects to Other Ideas

To understand greatest common factor, you should first be comfortable with factors and divisibility intuition. Once you have a solid grasp of greatest common factor, you can move on to simplification and least common multiple.

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