If a and b are relatively prime no then , their HCF is 1 .
Because , HCF means highest common factor . no two prime numbers will have common factor greater than 1.
CHECK IT ONCE:
For eg , take two prime numbers 23 and 2,
they have one and only common factor , that is 1. It has no other common factor. Then we no need to bother about the highest one. since there is only one common factor 1