If mn=1, then m/1=1/n. But for n an integrer, then m is a fraction. The only way both are integers is if n=1 so that m=1, or if n=-1 so that m=-1.Prove or disprove that if m and n are integers such that mn = 1 then either m= 1 & n = 1 or else m = -1 & n = -1.