1. \(S=1+3+3^2+....+3^{98}\)
\(\Leftrightarrow S=\left(1+3+3^2\right)+\left(3^4+3^5+3^6\right)+....+\left(3^{96}+3^{97}+3^{98}\right)\)
\(\Leftrightarrow S=13+3^4.\left(1+3+3^2\right)+...+3^{96}.\left(1+3+3^3\right)\)
\(\Leftrightarrow S=13+3^4.13+...+3^{96}.13\)
\(\Leftrightarrow S=13.\left(1+3^4+...+3^{96}\right)⋮13\) ( đpcm )
1.
S = 1+ 3 + 32 + 33 +... + 398
S = (1 + 3 + 32) + (33 + 34 + 35) + ... + (396 + 397 + 398)
S = (1 + 3 + 32) + 33(1 + 3 + 32) + ... + 396(1 + 3 + 32)
S = 13 + 32 . 13 + ... + 396 . 13
S = 13 (1 + 32 + ... + 396) ⋮ 13 (đpcm)