Implement: (1) GCD using Euclidean algorithm (iterative + recursive), (2) Extended Euclidean: find x,y such that ax+by=gcd(a,b), (3) Modular multiplicative inverse using Fermat's Little Theorem: a^(p-2) mod p, (4) Modular exponentiation (fast pow): a^b mod m in O(log b), (5) Solve linear congruence: find x in ax ≡ b (mod m). Test with large numbers (10^9+7 modulus).
