Can you help me with this problem: if a^2 divides b^2, show that a divides b. This was a homework question that I had a while ago and it was solved by using the fundamental theorem of arithmetic. I instead tried to solve it with proof by contradiction: a^2 divides b^2 implies a divides b^2 counterassumption: a does not divide b => b = q*a + r, 0< r < a Now b^2 = q^2*a^2 + 2qar + r^2, now a divides q^2*a^2 and 2qar, but what about r^2. At this part I got confused, and went out for a beer. Can you help with this?