1.

i) if p is prime, a,b are integers, and p divdes ab, then p divides a or p divides b[Hint use result when proving the theorem there exists integers m, n such that g = m*a=n*b]

ii) If r is a rational number then there exist integers m,n such that n>0, r = m/n and m, n have no common factors.. [USE WELL ORDERING PRINCIPLE]

iii) PROVE SQRT(21) is irrational [Hint use part 1 and part 2]