In the realm of number theory, the Highest Common Factor (HCF) stands as a key concept, holding the power to unlock the secrets of divisibility and commonality among numbers. In this comprehensive guide, we delve into the definition of HCF and unravel the methods to find it, providing you with a thorough understanding of this fundamental mathematical principle.

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**Definition of Highest Common Factor**

The Highest Common Factor, also known as the Greatest Common Divisor (GCD), is the largest positive integer that divides two or more numbers without leaving a remainder. Symbolized as \(\text{HCF}(a, b)\) for two numbers a and b, or \(\text{HCF}(a, b, c)\) for three numbers a, b, and c, it represents the greatest shared factor among the given integers.

**What is HCF?**

The Highest Common Factor (HCF), also known as the Greatest Common Divisor (GCD), is a fundamental concept in number theory. It represents the largest positive integer that divides two or more numbers without leaving a remainder. For a set of integers, the HCF is the greatest number that is a factor of each integer in the set.

**Calculation:** The HCF is calculated by identifying the common factors of the given numbers and selecting the greatest one. Factors are the positive integers that divide a number without leaving a remainder.

**Example:** Let’s find the HCF of 24 and 36.

- Factors of 24: 1, 2, 3, 4, 6, 8, 12, 24
- Factors of 36: 1, 2, 3, 4, 6, 9, 12, 18, 36

The common factors are 1, 2, 3, 4, 6, 12. The greatest common factor (HCF) is 12.

**Why HCF Matters**

Understanding the HCF is crucial in various mathematical applications. It plays a pivotal role in simplifying fractions, solving Diophantine equations, and designing algorithms, particularly the Euclidean Algorithm, renowned for its efficiency in finding the HCF.

**How to Find HCF:**

**Listing Factors:**- Begin by listing the factors of each number.
- Example: To find the HCF of 24 and 36, list the factors of 24 (1, 2, 3, 4, 6, 8, 12, 24) and 36 (1, 2, 3, 4, 6, 9, 12, 18, 36).

**Identifying Common Factors:**- Identify the common factors shared by both numbers.
- Example: Common factors of 24 and 36 are 1, 2, 3, 4, 6, 12.

**Selecting the Greatest Common Factor:**- Choose the greatest common factor as the HCF.
- Example: The HCF of 24 and 36 is 12.

**Properties of HCF:**

- The HCF of any two numbers is a divisor of their difference.
- The product of the HCF and the least common multiple (LCM) of two numbers is equal to the product of the numbers.

**Relation Between LCM and HCF**

The relationship between the Least Common Multiple (LCM) and the Highest Common Factor (HCF) is encapsulated in a fundamental mathematical principle known as Bézout’s identity. This relationship is succinctly expressed through Bézout’s Lemma, which states that for any two integers a and b, there exist integers x and y such that:

\(ax + by\) = \(\text{HCF}(a, b) \times \text{LCM}(a, b)\)

This relationship underscores the interplay between the HCF and LCM, illustrating that their product is equal to the product of the original numbers (a and b) and their linear combination. In other words, the HCF and LCM are interconnected through a mathematical equation that involves the coefficients x and y.

#### What is the HCF of 2, 5, and 18?

The highest common factor (HCF) of 2, 5, and 18 is 1. This is because there are no common factors other than 1 that divide all three numbers evenly.

**HCF Examples**

**Example 1: Finding HCF of 18 and 24**

- List the factors of 18: 1, 2, 3, 6, 9, 18.
- List the factors of 24: 1, 2, 3, 4, 6, 8, 12, 24.
- Identify the common factors: 1, 2, 3, 6.
- Choose the greatest common factor as the HCF: HCF(18, 24) = 6.

**Example 2: Finding HCF of 42 and 56**

- List the factors of 42: 1, 2, 3, 6, 7, 14, 21, 42.
- List the factors of 56: 1, 2, 4, 7, 8, 14, 28, 56.
- Identify the common factors: 1, 2, 7, 14.
- Choose the greatest common factor as the HCF: HCF(42, 56) = 14.

**Example 3: Finding HCF of 60 and 75**

- List the factors of 60: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60.
- List the factors of 75: 1, 3, 5, 15, 25, 75.
- Identify the common factors: 1, 3, 5, 15.
- Choose the greatest common factor as the HCF: HCF(60, 75) = 15.