Question

[Nguyễn Thùy Chi]

1+1/3+1/6+1/10+......+2/x×(x+1)=4032/2017

Answer 1

[Nguyễn Lê Phước Thịnh]

\(\Leftrightarrow\dfrac{2}{2}+\dfrac{2}{6}+\dfrac{2}{12}+\dfrac{2}{20}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{4032}{2017}\)

\(\Leftrightarrow2\left(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+...+\dfrac{1}{x\left(x+1\right)}\right)=\dfrac{4032}{2017}\)

\(\Leftrightarrow1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{x}-\dfrac{1}{x+1}=\dfrac{2016}{2017}\)

\(\Leftrightarrow1-\dfrac{1}{x+1}=\dfrac{2016}{2017}\)

\(\Leftrightarrow\dfrac{1}{x+1}=\dfrac{1}{2017}\)

=>x+1=2017

hay x=2016