2 điểm!?

thi hay sao?

S= 1/1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729

S= 3 x ( 1/1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729 )

S = 3 +1 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729

S= 3 +1 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729 - 1 - 1/9 -1/27 - 1/81 - 1/243 - 1/729

S = 3 - 1/729 

S= 142/729

= 1 x 27/3x27 + 1x9/9x9 + 1x3 / 27 x 3 + 1/81

=27/81 + 9/81 + 3/81 + 1/81

= 40/81

\(3S=241+81+27+9+...+\dfrac{1}{9}+\dfrac{1}{27}\)

\(2S=3S-S=241-\dfrac{1}{81}=\dfrac{241x81-1}{81}\)

\(\Rightarrow S=\dfrac{241x81-1}{2x81}\)

\(=1\frac{364}{729}\)\(=1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}=1+\frac{243}{729}+\frac{81}{729}+\frac{27}{729}+\frac{9}{729}+\frac{3}{729}+\frac{1}{729}=1\frac{ }{ }\)

\(=1\frac{364}{729}\)

Câu trả lời hay nhất: Đặt A = 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729
A x 3 = 3 x ﴾1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729﴿
= 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243
A x 3 ‐ A = 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 ‐ ﴾1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729﴿
= 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 ‐ 1/3 ‐ 1/9 ‐ 1/27 ‐ 1/81 ‐ 1/243 ‐ 1/729
= 1 ‐ 1/729
A x 2 = 728/729
A = 364/729

NHỚ TK MK NHA

1 + 1/3 + 1/9+1/27+1/81+1/243+1/729

=1+1-1/3+1/3-1/9+1/9-1/27-1/27-1/81+1/81-1/243

= 2 - 1/243

=485/243

  \(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\)

=\(1+\frac{1}{3^1}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+\frac{1}{3^5}+\frac{1}{3^6}\)

=\(\frac{3^6}{3^6}+\frac{3^5}{3^6}+\frac{3^4}{3^6}+\frac{3^3}{3^6}+\frac{3^2}{3^6}+\frac{3^1}{3^6}+\frac{3^0}{3^6}\)

=\(\frac{3^6+3^5+3^4+3^3+3^2+3+1}{3^6}\)

=\(\frac{729+243+81+27+9+3}{729}\)

=\(\frac{1093}{729}\)

nha.

tong cua day so tren la 1093/729

1+1/3+1/9+1/27+1/81+1/243+1/729

=729/729+243/729+81/729+27/729+9/729+3/729+1/729

=1093/729

S = 1/3+1/9+1/27+1/81+1/243+1/729+1/2187 ( 1 ) 
Nhân S với 3. Ta có: 
S x 3 = 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729 ( 2 ) 
Trừ ( 2 ) với ( 1 ) ta có: 
S x 3 - S = 1 - 1/ 2187 
2S = 2186/ 2187 
S = 2186/ 2187 : 2 
S = 1093/ 2187