S = 1 + 2 + 2^2 + 2^3 + 2^4 + 2^5 + 2^6 + 2^7
S = (1+2) + (2^2 + 2^3) + (2^4 + 2^5) + (2^6 + 2^7)
S = (1+2) + 2^2 (1+2) + 2^4 (1+2) + 2^6 (1+2)
S = 3*1 + 2^2 * 3 + 2^4 * 3 + 2^6 * 3
S = 3 * (1 + 2^2 + 2^4 + 2^6)
Vì 3 ⁝ 3
nên 3 * (1 + 2^2 + 2^4 + 2^6) ⁝ 3
Vậy S ⁝ 3
S = 1 + 2 + 22 + 23 + 24 + 25 + 26 + 27
S = (1 + 2) + (22 + 23) + (24 + 25) + (26 + 27)
S = 1(1 + 2) + 22(1 + 2) + 24(1 + 2) + 26(1 + 2)
S = (1 . 3) + (22 . 3) + (24 . 3) + (26 . 3)
S = 3 . (1 + 22 + 24 + 26) ⋮ 3
S ⋮ 3