Question

C=\(\dfrac{2}{1x2}\)+\(\dfrac{2}{2x3}\)+\(\dfrac{2}{3x4}\)+...+\(\dfrac{2}{2018x2019}\)+\(\dfrac{2}{2019x2020}\)

Answer 1

\(C=\dfrac{2}{1\times2}+\dfrac{2}{2\times3}+...+\dfrac{2}{2019\times2020}\)

\(=2\left(\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+...+\dfrac{1}{2019\times2020}\right)\)

\(=2\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{2019}-\dfrac{1}{2020}\right)\)

\(=2\left(1-\dfrac{1}{2020}\right)=2.\dfrac{2019}{2020}=\dfrac{2019}{1010}\)

Answer 2

lớp 5 đây á

no no

đây ko phải lớp 5 mọi người nhỉ ?