(1+1/1.3).(1+1/2.4).(1+1/3.5)....(1+1/2019.2021) giúp mình với mọi người ơi

`( 1 + 1/( 1 . 3 ) ) . ( 1 + 1/( 2 . 4 ) ) . ( 1 + 1/( 3 . 5 ) ) ..... ( 1 + 1/( 2019 . 2021 ) )`

`=( ( 1 . 3 ) + 1 )/( 1 . 3 ) . ( ( 2 . 4 ) + 1 )/( 2 . 4 ) . ( 3 . 5 + 1 )/( 3 . 5 ) .... ( 2019 . 2021 + 1 )/( 2019 . 2021 )`

`=( 1 + 1 )^2/( 1 . 3 ) . ( 2 + 1 )^2/( 2 . 4 ) . ( 3 + 1 )^2/( 3 . 5 ) .... ( 2019 + 1 )^2/( 2019 . 2021 )`

`=2^2/( 1. 3 ) . 3^2/( 2 . 4 ) . 4^2/( 3 . 5 ) .... ( 2020 )^2/( 2019 . 2021 )`

`=( 2^2 . 3^2 . 4^2 .... 2020^2 )/( 1 . 3 . 2 . 4 . 3 . 5 .... 2019 . 2021 )`

`=( ( 2 . 3 . 4 .... 2020 ) . ( 2 . 3 . 4 ... 2020 ) )/( ( 1 . 2 . 3 .... 2019 ) . ( 3 . 4 . 5 ... 2021 ) )`

`=( 2020 . 2 )/( 1 . 2021 )`

`=4040/2021`