Question

[bong]

Bài 19: Tinh tổng sau :

D = 1/18 + 1/54 + 1/108 + .....+ 1/990

Answer 1

[BAN is VBN]

\(D=\frac{1}{18}+\frac{1}{54}+\frac{1}{108}+...+\frac{1}{990}\)

\(\Rightarrow D=\frac{1}{3\cdot6}+\frac{1}{6\cdot9}+\frac{1}{9\cdot12}+...+\frac{1}{30\cdot33}\)

\(\Rightarrow D=\frac{1}{3}\left(\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{12}+...+\frac{1}{30}-\frac{1}{33}\right)\)

\(\Rightarrow D=\frac{1}{3}\left(\frac{1}{3}-\frac{1}{33}\right)=\frac{1}{3}\cdot\frac{10}{33}=\frac{10}{99}\)

Answer 2

[BAN is VBN]

k ủng hộ nha!!!!!!!!