\(A=\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27}+\dfrac{1}{81}+\dfrac{1}{243}\\ =\dfrac{1}{3}+\dfrac{1}{3^2}+\dfrac{1}{3^3}+\dfrac{1}{3^4}+\dfrac{1}{3^5}\\ =>3A=1+\dfrac{1}{3}+\dfrac{1}{3^2}+\dfrac{1}{3^3}+\dfrac{1}{3^4}\\ =>3A-A=2A=1-\dfrac{1}{3^5}\\ =>A=\dfrac{1-\dfrac{1}{3^5}}{2}=\dfrac{3^5-1}{2.3^5}\)

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

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

3A-A=(1+1/3+1/9+1/27+1/81+1/243)-(1/3+1/9+1/27+1/81+1/243+1/729)

3A-A=1-1/3+1/3-1/9+1/9-1/27+1/27-1/81+1/81-1/243+1/243-1/729)

2A=1-1/729

2A=728/729

A=728/729/2

A=364/729

Chúc bạn học tốt :))

\(3A=3\cdot\left(\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27}+\dfrac{1}{81}+\dfrac{1}{243}+\dfrac{1}{729}\right)\)

\(\Rightarrow3A=1+\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27}+\dfrac{1}{81}+\dfrac{1}{243}\)

Lấy \(3A-A=1-\dfrac{1}{729}\)

\(\Rightarrow2A=\dfrac{728}{729}\Rightarrow A=\dfrac{364}{729}\)

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

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

3A-A=(1+1/3+1/9+1/27+1/81+1/243)-(1/3+1/9+1/27+1/81+1/243+1/729)

3A-A=1-1/3+1/3-1/9+1/9-1/27+1/27-1/81+1/81-1/243+1/243-1/729)

2A=1-1/729

2A=728/729

A=728/729/2

A=364/729

Ta có:  1/3 = 27/81
            1/9 = 9/81
            1/27 = 3/81
Thay các giá trị vừa quy đồng vào A ta có:
A = 27/81 + 9/81 + 3/81 + 1/81
A = 27 + 9 + 3 + 1
            81
A = 40
      81

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

\(A=\frac{4}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\)

\(A=\frac{12}{9}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\)

Từ chỗ này làm dễ hơn rồi bạn tự làm tiếp đi nhé

1093\729

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

Mọi người giúp mình nha!

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

(y + \(\dfrac{1}{3}\)) + ( y + \(\dfrac{1}{9}\)) + ( y + \(\dfrac{1}{27}\)) + ( y + \(\dfrac{1}{81}\)) = \(\dfrac{56}{81}\)

( y + y + y + y ) + (\(\dfrac{1}{3}\)+ \(\dfrac{1}{9}\) + \(\dfrac{1}{27}\) + \(\dfrac{1}{81}\)) = \(\dfrac{56}{81}\)

4\(y\) + ( \(\dfrac{27}{81}\) + \(\dfrac{9}{81}\) + \(\dfrac{3}{27}\) + \(\dfrac{1}{81}\) ) = \(\dfrac{56}{81}\)

4y + \(\dfrac{40}{81}\) = \(\dfrac{56}{81}\)

4y = \(\dfrac{56}{81}\) - \(\dfrac{40}{81}\)

4y = \(\dfrac{16}{81}\)

y  = \(\dfrac{16}{81}\) : 4

y = \(\dfrac{4}{81}\)

\(\left(y+\dfrac{1}{3}\right)+\left(y+\dfrac{1}{9}\right)+\left(y+\dfrac{1}{27}\right)+\left(y+\dfrac{1}{81}\right)=\dfrac{56}{81}\)

\(\Rightarrow y+\dfrac{1}{3}+y+\dfrac{1}{9}+y+\dfrac{1}{27}+y+\dfrac{1}{81}=\dfrac{56}{81}\)

\(\Rightarrow4\times y+\dfrac{40}{81}=\dfrac{56}{81}\)

\(\Rightarrow4\times y=\dfrac{56}{81}-\dfrac{40}{81}\)

\(\Rightarrow4\times y=\dfrac{16}{81}\)

\(\Rightarrow y=\dfrac{16}{81}:4\)

\(\Rightarrow y=\dfrac{4}{81}\)

\((y+\dfrac{1}{3})+(y+\dfrac{1}{9})+(y+\dfrac{1}{27})+(y+\dfrac{1}{81})=\dfrac{56}{81}\)

\(=>4y+(\dfrac{1}{3}+\dfrac{1}{9]+\dfrac{1}{27}+\dfrac{1}{81})=\dfrac{56}{81}\)

\(=>4y+\dfrac{1}{3}(1+\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27})=\dfrac{56}{81}\)

\(=>4y+\dfrac{1}{2}(1+\dfrac{1}{3}+\dfrac{1}{9}+\dfrac{1}{27}-\dfrac{1}{3}-\dfrac{1}{9}-\dfrac{1}{27}-\dfrac{1}{81})=\dfrac{56}{81}\)

\(=>4y+\dfrac{1}{2}(1-\dfrac{1}{81})=\dfrac{56}{81}\)

\(=>4y+\dfrac{1}{2}\times\dfrac{80}{81}=\dfrac{56}{81}\)

\(=>4y+\dfrac{40}{81}=\dfrac{56}{81}\)

\(=>4y=\dfrac{56}{81}-\dfrac{40}{81}\)

\(=>4y=\dfrac{16}{81}\)

\(=>y=\dfrac{4}{81}\)

A = 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/59049

3 x A = 3 x ( 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/59049 )

3 x A = 1 + 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/19683 

3 x A - A = 1 + 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/19683 

- ( 1 + 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/59049 )

= 1 - 1/59049 

2 x A = 59048/59049

A = 59048/59049 : 2

A = 29524/59049

A = 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/59049

3 x A = 3 x ( 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/59049 )

3 x A = 1 + 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/19683 

3 x A - A = 1 + 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/19683 - ( 1 + 1/3 + 1/9 + 1/27 + 1/81 + ... + 1/59049 )

= 1 - 1/59049 

2 x A = 59048/59049

A = 59048/59049 : 2

A = 29524/59049

(a+\(\dfrac{1}{1.3}\))+(a+\(\dfrac{1}{3.5}\))+(a+\(\dfrac{1}{5.7}\))+..+(a+\(\dfrac{1}{23.25}\))=11.a+(\(\dfrac{1}{3}\)+\(\dfrac{1}{9}\)+\(\dfrac{1}{27}\)+\(\dfrac{1}{81}\)+\(\dfrac{1}{243}\))

(a+a+..+a)+(\(\dfrac{1}{1.3}\)+\(\dfrac{1}{3.5}\)+\(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{23.25}\)) = 11.a+ \(\dfrac{1}{3}\)+\(\dfrac{1}{9}\)+\(\dfrac{1}{27}\)+\(\dfrac{1}{81}\)+\(\dfrac{1}{243}\))

Đặt A =(a+a+..+a) + \(\dfrac{1}{1.3}\)+\(\dfrac{1}{3.5}\)+\(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{23.25}\)

Xét dãy số 1; 3; 5;...;25 Dãy số trên là dãy số cách đều với khoảng cách là: 3-1 = 2

Dãy số trên có số số hạng là: (25 - 1): 2 + 1  = 13

Vậy A = a\(\times\)13 + \(\dfrac{1}{1.3}\)+\(\dfrac{1}{3.5}\)+\(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{23.25}\)

A = a\(\times\)13 + \(\dfrac{1}{2}\) \(\times\)(\(\dfrac{2}{1.3}\)+\(\dfrac{2}{3.5}\)+\(\dfrac{2}{5.7}\)+...+\(\dfrac{2}{23.25}\))

A = a \(\times\) 13 + \(\dfrac{1}{2}\times\)( \(\dfrac{1}{1}-\dfrac{1}{3}\)+\(\dfrac{1}{3}\)-\(\dfrac{1}{5}\)+\(\dfrac{1}{5}\)- \(\dfrac{1}{7}\)+...+\(\dfrac{1}{23}\) - \(\dfrac{1}{25}\))

A = a\(\times\)13 + \(\dfrac{1}{2}\) \(\times\) \(\dfrac{24}{25}\)

A = a\(\times\)13 + \(\dfrac{12}{25}\) (1)

Đặt B =    \(\dfrac{1}{3}\) + \(\dfrac{1}{9}\)+ \(\dfrac{1}{27}\)+\(\dfrac{1}{81}\)+\(\dfrac{1}{243}\)

B\(\times\)3 =1 + \(\dfrac{1}{3}\)+\(\dfrac{1}{9}\)+\(\dfrac{1}{27}\)+\(\dfrac{1}{81}\)

B\(\times\)3 - B = 1 - \(\dfrac{1}{243}\) = \(\dfrac{242}{243}\)

2B = \(\dfrac{242}{243}\)

B = \(\dfrac{242}{243}\): 2

B = \(\dfrac{121}{243}\)

11a + B = 11a + \(\dfrac{121}{243}\) (2)

Từ (1) và(2) ta có:

a\(\times\)13  + \(\dfrac{12}{25}\) = 11\(\times\) a + \(\dfrac{121}{143}\)

a \(\times\) 13 + \(\dfrac{12}{25}\) - 11 \(\times\)a = \(\dfrac{121}{143}\) 

\(a\times\)(13 - 11) + \(\dfrac{12}{25}\) = \(\dfrac{121}{143}\)

a \(\times\) 2 + \(\dfrac{12}{25}\) = \(\dfrac{121}{243}\)

a \(\times\) 2 = \(\dfrac{121}{243}\) - \(\dfrac{12}{25}\)

a \(\times\) 2 = \(\dfrac{109}{6075}\)

a = \(\dfrac{109}{6075}\): 2

a = \(\dfrac{109}{12150}\)