B = 3 + 32 + 33 + ... + 337 + 338 + 339
=> B = 3 . (1 + 3 + 32) + ... + 337 . (1 + 3 + 32)
=> B = 3 . (1 + 3 + 9) + ... + 337 . (1 + 3 + 9)
=> B = 3 . 13 + ... + 337 . 13
=> B = 13 . (3 + ... + 337) \(⋮\)13 (đpcm)
B = 3 + 32 + 33 + 34 +...+339
B = ( 3 + 32 + 33) + (34 + 35 + 36) + ...+ (337 + 338 + 339)
B = 3. (1 + 3 + 32) + 34. (1 + 3 + 32) +...+ 337. (1 + 3 + 32)
B = 3.13 + 34 . 13 +... + 337. 13
B = 13. ( 3 + 34 +...+ 337) \(⋮\)13
Vậy B \(⋮\)13
Chứng tỏ rằng:
B = 3 + 32 + 33 + 34 + ... + 339 ⋮ 13
Giải:
B = 3.(1 + 3 + 32) + 34.(1 + 3 + 32) + ... + 337.(1 + 3 + 32)
B = 3 . 13 + 34 . 13 + ... + 337 . 13
B = 13.(3 + 34 + ... + 337)
=> B ⋮ 13.
