`1/(1.2.3)+1/(2.3.4)+...+1/(2019.2020.2021)` `=1/2(2/(1.2.3)+2/(2.3.4)+...+2/(2019.2020.2021))` `=1/2(1/1.2-1/2.3+1/2.3-1/3.4+...+1/2019.2020+1/2020.2021)` `=1/2(1/2-1/2020.2021)` `=1/2.(2041210/2020.2021-1/2020.2021)`(đến đây có thể ko tính nx bạn nhé nếu rút gọn) `=1/2. 201209/4082420` `=201209/8164540`