Question

[nguyen thi le na]

Bài:1

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

C = 1/3 + 1/9 + 1/27 + 1/81 + ............+ 1/59040

A= 2/3x5 + 2/7x9 + 2/9x11 + 2/11x13 + 2/13x15 + 2/1x2 + 2/2x3 + 2/3x4 + 2/4x5 + .............+ 2/8x9 + 2/9x10

B = 4/3X6 + 4/ 6X9 + 4/ 9X12 + 4/ 12X15 + 4/15x18

C = 1/10 + 1/40 + 1/88 + 1/154 + 1/238 + 1/345

bài:2 .Cho tổng.

S = 4/3X7 + 4/7X11 + 4/11/15 + ..........= 664/ 1995

Tìm số hạng cuối cùng của dãy S

Tổng S có bao nhiêu số hạng

Answer 1

[nguyen thi le na]

giup minh voi 

Answer 2

[Lê Jiabao]

Ta có: \(B=\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\)

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

\(\Rightarrow3B=1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+\frac{1}{3^5}\)

\(\Rightarrow3B-B=\left(1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^4}+\frac{1}{3^5}\right)-\left(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+\frac{1}{3^5}+\frac{1}{3^6}\right)\)

\(\Rightarrow2B=1-\frac{1}{3^6}\)

\(\Rightarrow B=\frac{1-\frac{1}{3^6}}{2}\)