Question

Answer 1

tinh nhanh nha mn

Answer 2

\(=2\left(\dfrac{2}{2\cdot4}+\dfrac{2}{4\cdot6}+...+\dfrac{2}{18\cdot20}\right)\)

\(=2\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+...+\dfrac{1}{18}-\dfrac{1}{20}\right)\)

\(=2\cdot\dfrac{9}{20}=\dfrac{9}{10}\)