\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{2020x2021}\)

\(=1-\dfrac{1}{2021}=\dfrac{2020}{2021}\)

lx:v

a) \(\dfrac{5}{6}-\dfrac{7}{12}\)

\(=\dfrac{10}{12}-\dfrac{7}{12}\)

\(=\dfrac{1}{4}\)

b) \(2:\dfrac{4}{5}\)

\(=2\cdot\dfrac{5}{4}\)

\(=\dfrac{5}{2}\)

c) \(\dfrac{5}{7}+\dfrac{3}{7}\cdot\dfrac{1}{2}\)

\(=\dfrac{5}{7}+\dfrac{6}{14}\cdot\dfrac{7}{14}\)

\(=\dfrac{5}{7}+\dfrac{3}{14}\)

\(=\dfrac{10}{14}+\dfrac{3}{14}\)

\(=\dfrac{13}{14}\)

d) \(\left(\dfrac{1}{3}-\dfrac{1}{5}\right):\dfrac{2}{3}\)

\(=\left(\dfrac{5}{15}-\dfrac{3}{15}\right):\dfrac{2}{3}\)

\(=\dfrac{2}{15}\cdot\dfrac{3}{2}\)

\(=\dfrac{1}{5}\)

đề?