You learned what GCD and LCM mean and how to compute them efficiently. The Euclidean algorithm finds GCD in O(log min(a, b)) time by repeatedly replacing (a, b) with (b, a mod b).
You saw the formula lcm(a, b) = (a × b) / gcd(a, b) and how to use inclusion-exclusion to count numbers divisible by multiple values. You walked through three problems using GCD and LCM with binary search.
You now recognize when a problem needs GCD (common factors, simplification) or LCM (synchronization, cycles). Next section: modular arithmetic and how to compute with remainders.