If a and b are coprime and c is a factor of a, then prove that b and c too are coprime?

1 Answer
Mar 18, 2017

Please see below.

Explanation:

Numbers whose GCD is 1 are known as coprime or relatively prime.

If two numbers are prime numbers, they will be coprime. But even if there are two composite numbers, let us say 25 and 36, as there is no common factor between them, they are coprime and their GCD is 1.

Now as GCD of a and b is 1, there is no common factor between them.

We have c, which is a factor of a, but as there is no common factor between a and b,

there will not be a common factor between b and c, as otherwise this common factor would have been GCD of a and b, (as it divides both).

Hence, GCD of b and c too is 1.

As an example, GCD of 36 and 25 is 1, but though 12 is a factor of 36, GCD of 12 and 25 too is 1