Question

4/8×13+4/13×18+4/18×23+...+4/253×258

Answer 1

đặt A là ...

\(\Leftrightarrow\dfrac{5}{4}A=\dfrac{5}{8.13}+\dfrac{5}{13.18}+\dfrac{5}{18.24}+...+\dfrac{5}{253.258}\)

\(\Leftrightarrow\dfrac{5}{4}A=\dfrac{13-8}{8.13}+\dfrac{18-13}{13.18}+...+\dfrac{258-253}{253.258}\)

\(\Leftrightarrow\dfrac{5}{4}A=\dfrac{1}{8}-\dfrac{1}{13}+\dfrac{1}{8}-\dfrac{1}{13}+...+\dfrac{1}{253}-\dfrac{1}{258}\)

\(\Leftrightarrow\dfrac{5}{4}A=\dfrac{1}{8}-\dfrac{1}{258}\)

\(\Leftrightarrow\dfrac{5}{4}A=\dfrac{250}{258.8}\)

\(\Rightarrow A=\dfrac{250}{258.8}.\dfrac{4}{5}=\dfrac{25}{258}\)

Answer 2

https://i.imgur.com/MA32pns.jpg