Ta có M = 2 + 22 + 23 + 24 + 25 + 26 + 27 + 28 + ... + 217 + 218 + 219 + 220
= (2 + 22 + 23 + 24) + (25 + 26 + 27 + 28) + ... + (217 + 218 + 219 + 220)
= 2(1 + 2 + 22 + 23) + 25(1 + 2 + 22 + 23) + ... + 217(1 + 2 + 22 + 23)
= (1 + 2 + 22 + 23)(2 + 25 + ... + 217)
= 15(2 + 25 + ... + 217)
= 3.5.(2 + 25 + ... + 217)
=> M \(⋮\)3;5