(31+ 32) + ( 33+ 34) +..........................................+(329+330)
= 3( 1+ 3) + 33( 1+ 3).........................................+ 329(1 + 3)
=3 x4 + 32x 4 + .............................................329x 4
= 4x(3 + 32+.........................................329) chia hết cho 4
Gọi A = 3 + 32 + 33 +34 + ... + 327 + 328 + 329 + 330
= (3 + 32) + (33 +34) + ... + ( 327 + 328) + (329 + 330)
= 1.(3+32) + 32.(3 + 32) + ... + 326.(3 + 32) + 328.(3 + 32)
= 1. 12 + 32. 12 + ... + 326. 12 + 328. 12
= 12.(1 + ... + 326 + 328)
Mà \(12⋮4\Rightarrow12.\left(1+...+3^{26}+3^{28}\right)\)
\(\Rightarrow A⋮4\)
Nhớ **** nhen!!!
