Tìm số tự nhiên x biết 1/3 +1/6 +1/10 +...+ 1/x(x+1) : 2 = 2019/2021

Đáp án cần chọn là: C

13+16+110+...+1x(x+1):2=201920212.[12.3+13.4+...+1x(x+1)]=201920212.(12−13+13−14+...+1x−1x+1)=201920212.(12−1x+1)=201920211−2x+1=201920212x+1=1−201920212x+1=22021x+1=2021x=2020