Ta có
` (1-\frac{1}{2}) \times (1-\frac{1}{3}) \times (1-\frac{1}{4}) ..... \times (1-\frac{1}{2020}) \times (1-\frac{1}{2021})`
` = \frac{1}{2} \times \frac{2}{3} \times \frac{3}{4} ..... \times \frac{2019}{2020} \times \frac{2020}{2021}`
` = \frac{1\times2\times3\times.....\times2019\times2020}{2\times3\times4\times....\times2020\times2021}`
` = \frac{1}{2021}`