Đáp án:
`D = 4040/2021`
Giải thích các bước giải:
`d, D = ( 1 + 1/1.3 ) . ( 1 + 1/2.4 ) . ( 1 + 1/3.5 ) ............ ( 1 + 1/2019.2021 )`
`D = 2^2/1.3 . 3^2/2.4 . 4^2/3.5 .............. 2020^2/2019.2021`
`D = (2.3..........2020)/(1.2.3...............2019) . (2.3.............2020)/(3.4.5..............2021)`
`D = 2020/1 . 2/2021`
`D = 4040/2021`