Question

khong quy dong hay tinh

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

Answer 1

 \(\text{Đ}\text{ặt}\text{ }A=1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}\)

\(\Rightarrow3A=3+1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}\)

\(\Rightarrow3A-A=3-\frac{1}{81}\)

\(\Rightarrow2A=\frac{242}{81}\)

\(\Rightarrow A=\frac{242}{81}:2=\frac{121}{81}\)