Ta có : A = 2 + 22 + 23 + 24 + .. + 259 + 260
= (2 + 22) + (23 + 24) + .. + (259 + 260)
= 2(2 + 1) + 23(2 + 1) + ... + 259(2 + 1)
= (2 + 1)(2 + 23 + ... + 259) = 3(2 + 23 + ... + 259) \(⋮\)3
giup minh voi
A = 2 + 22 + 23 + 24 + 25 + 26 + ... + 258 + 259 + 260
= (2 + 22 + 23) + (24 + 25 + 26) + ... + (258 + 259 + 260)
= 2(1 + 2 + 22) + 24(1 + 2 + 22) + ... + 258(1 + 2 + 22)
= (1 + 2 + 22)(2 + 24 + ... + 258)
= 7(2 + 24 + ... + 258) \(⋮\)7
A = 2 + 22 + 23 + 24 + 25 + 26 + 27 + 28 + ... + 257 + 258 + 259 + 260
= (2 + 22 + 23 + 24) + (25 + 26 + 27 + 28) + ... + (257 + 258 + 259 + 260)
= 2(1 + 2 + 22 + 23) + 25(1 + 2 + 22 + 23) + ... + 257(1 + 2 + 22 + 23)
= (1 + 2 + 22 + 23)(2 + 25 + ... + 257)
= 15(2 + 25 + ... + 257) = 3.5(2 + 25 + ... + 257)\(⋮\)5