Ok I am trying to solve this question.
Assume n \geq 3, prove that gcd(x, n) = gcd(n-x, n) for all 0 \leq x \leq n/2.
This is what i got:
x = 0 then gcd(0, n) = gcd(n-0, n) = n
x = n/2 then gcd(n/2, n) = gcd(n-n/2, n) = n/2
0 < x < n/2 then x = n/k for some k>2
gcd(n/k, kn/k)...