B = 3+32 +...+3100
=> B = (3+32+33+34)+(35+36+37+38)+.....+(397+398+399+3100)
=> B = 120 + 34 . 120 +......+396 . 120
=> B = 120.(1+34+38+....+396) chia hết cho 120
=> B chia hết cho 120
Cho Mình
B = 3+3^2+3^3+3^4+...+3^97+3^98+3^99+3^100
B = (3+3^2+3^3+3^4)+...+3^96.(3+3^2+3^3+3^4)
B = 120+...+3^96.120
B = 120.(1+...+3^96) => B chia hết cho 120 (đpcm)