S = 1+2+2^3+2^4+2^5+2^6+2^7
S = (1+2)+(2^4+2^5)+(2^6+2^7)
S = 1.3+2^4(1+2)+2^6(1+2)
S = 1.3+2^4.3+2^6.3
S = 3(1+2^4+2^6)
=> S chia hết cho 3
Sửa đề: Cho \(S=1+2+2^2+2^3+2^4+2^5+2^6+2^7\)
Chứng minh rằng S chia hết cho 3
\(S=1+2+2^2+2^3+2^4+2^5+2^6+2^7\)
\(S=\left(1+2\right)+\left(2^2+2^3\right)+\left(2^4+2^5\right)+\left(2^6+2^7\right)\)
\(S=3+2^2\left(1+2\right)+2^4\left(1+2\right)+2^6\left(1+2\right)\)
\(S=3+2^2.3+2^4.3+2^6.3\)
\(S=3\left(1+2^2+2^4+2^6\right)\)
Vì \(3 ⋮ 3\)
\(\Rightarrow3\left(1+2^2+2^4+2^6\right)⋮3\) ( đpcm)
Chúc bạn hok tốt!!! Lê Tấn Khải