Đáp án:
$B =\dfrac{1}{2021}$
Giải thích các bước giải:
$\quad B =\left(1-\dfrac12\right)\left(1-\dfrac13\right)\left(1-\dfrac14\right)\cdots\left(1-\dfrac{1}{2020}\right)\left(1-\dfrac{1}{2021}\right)$
$\to B = \dfrac{1}{2}\cdot \dfrac{2}{3}\cdot\dfrac{3}{4}\cdots \dfrac{2019}{2020}\cdot\dfrac{2020}{2021}$
$\to B =\dfrac{1}{2021}$