Question

1\5+1\10+1\20+1\40+….+1\1280

Answer 1

A = \(\dfrac{1}{5}+\dfrac{1}{10}+...+\dfrac{1}{1280}\)

= \(\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{20}+...+\dfrac{1}{640}-\dfrac{1}{1280}\)

= \(\dfrac{2}{5}-\dfrac{1}{1280}=\dfrac{511}{1280}\)

Answer 2

Giải:

\(\dfrac{1}{5}+\dfrac{1}{10}+\dfrac{1}{20}+\dfrac{1}{40}+...+\dfrac{1}{1280}\) 

\(=\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{20}+\dfrac{1}{20}-\dfrac{1}{40}+...+\dfrac{1}{640}-\dfrac{1}{1280}\) 

\(=\dfrac{2}{5}-\dfrac{1}{1280}\) 

\(=\dfrac{511}{1280}\)