A = 3 + 32 + 33 + 34 + ... + 349 + 350
A = ( 3 + 32 + 33 + 34 ) + ( 35 + 36 + 37 + 39 ) + ... + ( 347 + 348 + 349 + 350 )
A = 3 . ( 1 + 3 + 32 + 33 ) + 35 . ( 1 + 3 + 32 + 33 ) + ... + 347 . ( 1 + 3 + 32 + 33 )
A = 3 . 40 + 35 . 40 + ... + 347 . 40
A = 40 . ( 3 + 35 + ... + 347 ) \(⋮\)10
Vậy A \(⋮\)10
=> ( Đpcm )