@Đỗ Nguyễn Như Bình \(\frac{2}{3^2}\) hay là \(\frac{2^2}{3}\) hay là \(\left(\frac{2}{3}\right)^2\) vậy em???????????

ta có : 1+1+1+1+1+1+1+1x0

=> 1x8 = 8

mà kòn x vs 0 nữa :

=> tổng đó =0

=> 0<3/4

=> E<3/4

\(E=\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{100}{3^{100}}\)

\(3E=1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{100}{3^{99}}\)

\(3E-E=\left(1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{100}{3^{99}}\right)-\left(\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{100}{3^{100}}\right)\)

\(2E=1+\frac{1}{3}+\frac{1}{3^2}+....+\frac{1}{3^{99}}-\frac{100}{3^{100}}\)

\(6E=3+1+\frac{1}{3}+...+\frac{1}{3^{98}}-\frac{100}{3^{99}}\)

\(6E-2E=\left(3+1+\frac{1}{3}+...+\frac{1}{3^{98}}-\frac{100}{3^{99}}\right)-\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{99}}-\frac{100}{3^{100}}\right)\)

\(4E=3-\frac{100}{3^{99}}-\frac{1}{3^{99}}+\frac{100}{3^{100}}\)

\(4E=3-\frac{300}{3^{100}}-\frac{3}{3^{100}}+\frac{100}{3^{100}}\)

\(4E=3-\frac{203}{3^{100}}< 3\)

\(\Rightarrow4E< 3\)

\(\Rightarrow E< \frac{3}{4}\left(đpcm\right)\)

Bài 1:

Ta có: \(3+3^2+3^3+...+3^{100}\)

\(=\left(3+3^2+3^3+3^4\right)+....+\left(3^{97}+3^{98}+3^{99}+3^{100}\right)\)

\(=120+3^5\left(3+3^2+3^3+3^4\right)+....+3^{96}\left(3+3^2+3^3+3^4\right)\)

\(=120+3^5.120+...+3^{96}.120\)

\(=120.\left(1+3^5+.....+3^{96}\right)\)

\(\Rightarrow3+3^2+3^3+3^4+....+3^{100}\)chia hết cho 120 (vì có chứa thừa số 120)

1/3E=1/3^2+2/3^3+...+100/3^101

E-1/3E=1/3+1/3^2+1/3^3+...+1/3^100-1/3^101

2/3E=1/3+1/3^2+1/3^3+...+1/3^100-1/3^101

Đặt B=1/3+1/3^2+...+1/3^100

      1/3B=1/3^2+1/3^3+...+1/3^101

B-1/3B=1/3-1/3^101

2/3B=1/3-1/3^101

mà 1/3-1/3^101<1/3

=>2/3B<1/3

=>B<1/2

thay B vào E ta có

2/3E=B-1/3^101

Mà B-1/3^101<B

=>2/3E<B

Mà B<1/2

=>2/3E<1/2

=>E<3/4

k cho mk nha

Ta có:

3! chia hết cho 3.

4! chia hết cho 3.

........

100! chia hết cho 3.

Vì mỗi số trên chia hết cho 3 nên tổng chia hết cho 3.

Lại có:

1!=1

2!=1.2=2

1!+2!=3 chia hết cho 3.

Vậy E   .......................

Chúc em học tốt^^

thanks nha chi