Hi, thanks for taking the time to look in this thread. Here are the GCD (great common divisior) questions that need proof:
1. Let a and b be integers. Prove that gcd(a, b)|gcd(a + b, a − b).
2. Let a, b, c be integers. Suppose that gcd(a, b) = 1 and c|(a+b). Prove
that gcd(a, c) = 1.