GCD & LCM：
1.https://codeforces.com/contest/2126/problem/E
2.https://codeforces.com/contest/2123/problem/F
3.https://codeforces.com/contest/2091/problem/E
推公式：
1.https://codeforces.com/problemset/problem/1/A
2.https://codeforces.com/contest/2091/problem/D
3.https://atcoder.jp/contests/abc415/tasks/abc415_d
4.https://codeforces.com/contest/2093/problem/C
5.https://codeforces.com/contest/1985/problem/E
6.https://codeforces.com/contest/2009/problem/A
7.https://codeforces.com/contest/2121/problem/G
8.https://codeforces.com/contest/1931/problem/D
找规律：
1.https://codeforces.com/contest/2093/problem/A
2.https://codeforces.com/contest/2093/problem/B
组合数学：
1.https://codeforces.com/contest/1985/problem/G
