Question

How do you find the greatest common factor of $9$ and $21$ ?

Answer 1

Hint: There are various methods for finding the greatest common divisor of the given numbers. The simplest method to find the greatest common divisor is by prime factorization method. In the prime factorization method, we first represent the given two numbers as a product of their prime factors and then find the product of the lowest powers of all the common factors.

Complete step by step answer:
In the given question, we are required to find the highest common factor of $9$ and $21$. To find the highest common factor of the given numbers: $9$ and $21$, first we find the prime factors of all the numbers. We know that both the numbers given to us, $9$ and $21$, are composite numbers. Composite numbers are numbers that are divisible by a number other than one and the number itself. They have more than two factors.

So, we do the prime factorization of the numbers as,
Prime factors of $9$$ = 3 \times 3$
Prime factors of $21$$ = 3 \times 7$
Now, the greatest common divisor is the product of the lowest powers of all the common factors.Now, we can see that there is only one repeated factor,$3$ which is common in both the numbers. Also, the number $2$ is present only once in the prime factors of the numbers.Hence, greatest common factor of $9$ and $21$ $ = 3$

Hence, the greatest common factor (GCF) of $9$ and $21$ is $3$.

Note: Highest common factor or the greatest common divisor is the greatest number that divides both the given numbers. Similarly, the highest common factor can also be found by using the prime factorization method as well as using Euclid’s division lemma. Highest common divisor is just a product of common factors with lowest power.