The greatest common factor (GCF), also known as the highest common factor (HCF), is the largest positive integer that is a divisor of both given integers. In this article, we will delve into the concept of GCF and calculate the GCF of 18 and 45, using various methods and exploring its significance in mathematics.

Finding the GCF is a fundamental mathematical operation that finds applications in various fields, including number theory, algebra, and geometry. It plays a crucial role in simplifying fractions, finding least common multiples (LCM), and solving Diophantine equations. By understanding the GCF, we can gain insights into the relationships between numbers and their divisors.

To calculate the GCF of 18 and 45, we can utilize different approaches. Prime factorization, the Euclidean algorithm, and the difference of squares formula are commonly used methods. In the following sections, we will demonstrate these methods step by step, providing clear explanations and detailed examples.

Prime Factorization Method

The prime factorization method involves decomposing the given numbers into their prime factors. The prime factors are the numbers that cannot be further divided by any other numbers except 1 and themselves.

Prime factorization of 18: 18 = 2 x 32 = 2 x 3 x 3

Prime factorization of 45: 45 = 32 x 5 = 3 x 3 x 5

The GCF of 18 and 45 is the product of the common prime factors raised to the smallest exponents. In this case, the common prime factor is 3, which is raised to the power of 2. Therefore, the GCF of 18 and 45 using the prime factorization method is 3^2 = 9.

Euclidean Algorithm

The Euclidean algorithm is an iterative method that repeatedly subtracts the smaller number from the larger number until the remainder is 0. The last non-zero remainder is the GCF.

45 ÷ 18 = 2 remainder 9
18 ÷ 9 = 2 remainder 0

Therefore, the GCF of 18 and 45 using the Euclidean algorithm is 9.

Difference of Squares Formula

The difference of squares formula for finding the GCF of the form a^2 – b^2 is GCF(a^2 – b^2) = GCF(a + b, a – b).

In this case, 18 and 45 cannot be expressed as the difference of squares. Therefore, this method is not applicable for finding the GCF of 18 and 45.

Significance of GCF

The GCF has several significant applications in mathematics:

  • Simplifying fractions: The GCF is used to reduce fractions to their simplest form by dividing both the numerator and denominator by their GCF.
  • Finding LCM: The GCF and LCM (least common multiple) are inversely related. The LCM of two numbers is equal to the product of the numbers divided by their GCF.
  • Solving Diophantine equations: The GCF is used to find solutions to Diophantine equations, which are equations involving integers.


In summary, the GCF of 18 and 45 is 9. We demonstrated various methods to calculate the GCF, including prime factorization, the Euclidean algorithm, and the difference of squares formula. Understanding the concept of GCF is essential in mathematics, as it provides a foundation for solving more complex number problems.



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