1. The problem statement, all variables and given/known data 1. Prove that if ca=cb (mod m) and gcd(c,m)=1, then a=b (mod m) 2. Prove that if a=b (mod m), then gcd(a,m)=gcd(b,m) 2. Relevant equations 3. The attempt at a solution I can't figure out how to get started on these, especially the last one. Is it just a matter of expanding definitions?