[tex]\displaystyle \left(1-\dfrac{1}{2018}\right) \cdot \left(1-\dfrac{1}{2019}\right)\cdot \left(1-\dfrac{1}{2020}\right)=\\ \\\\ =\left(\dfrac{2018-1}{2018}\right) \cdot \left(\dfrac{2019-1}{2019}\right)\cdot \left(\dfrac{2020-1}{2020}\right)= \\ \\ \\ = \dfrac{2017}{2018}\cdot \dfrac{2018}{2019}\cdot \dfrac{2019}{2020} =\\ \\ \\ =\dfrac{2017}{2020}[/tex]