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.