D

\(lim\dfrac{2\sqrt{7n^2-2n}}{3n+2}=lim\dfrac{2\sqrt{n^2\left(7-\dfrac{2}{n}\right)}}{3n+2}=lim\dfrac{2n\sqrt{7-\dfrac{2}{n}}}{n\left(3+\dfrac{2}{n}\right)}\)

\(=lim\dfrac{2\sqrt{7-\dfrac{2}{n}}}{3+\dfrac{2}{n}}=\dfrac{2\sqrt{7}}{3}\) \(=\dfrac{a\sqrt{7}}{b}\) 

Suy ra : a/b = 2/3 => a - b = -1 

c b a b yên tâm đi mình học rồi

??? giúp j bạn

ko fai câu hỏi

???

=>2/6+2/12+2/20+...+2/x(x+1)=1010/1011

=>2(1/6+1/12+...+1/x(x+1))=1010/1011

=>1/2-1/3+1/3-1/4+...+1/x-1/x+1=505/1011

=>1/x+1=1/2022

=>x+1=2022

=>x=2021