câu a) làm rồi
b) \(\dfrac{1}{3\cdot5}+\dfrac{1}{5\cdot7}+\dfrac{1}{7\cdot9}+...+\dfrac{1}{37\cdot39}\)
\(=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{37}+\dfrac{1}{37}-\dfrac{1}{39}\)
\(=\dfrac{1}{3}-\dfrac{1}{39}\)
\(=\dfrac{4}{13}\)
c) \(\left(1-\dfrac{1}{2}\right)\cdot\left(1-\dfrac{1}{3}\right)\cdot\left(1-\dfrac{1}{4}\right)\cdot\cdot\cdot\left(1-\dfrac{1}{2022}\right)\)
\(=\left(\dfrac{2}{2}-\dfrac{1}{2}\right)\cdot\left(\dfrac{3}{3}-\dfrac{1}{3}\right)\cdot\left(\dfrac{4}{4}-\dfrac{1}{4}\right)\cdot\cdot\cdot\left(\dfrac{2022}{2022}-\dfrac{1}{2022}\right)\)
\(=\dfrac{1}{2}\cdot\dfrac{2}{3}\cdot\dfrac{3}{4}\cdot\cdot\cdot\dfrac{2021}{2022}\)
\(=\dfrac{1}{2022}\)
b) =13−15+15−17+17−19+19−137+137−139=13−15+15−17+17−19+19−137+137−139
=413=413
c) =(22−12)⋅(33−13)⋅(44−14)⋅⋅⋅(20222022−12022)=(22−12)⋅(33−13)⋅(44−14)⋅⋅⋅(20222022−12022)
=12022
