1+2+..+n=n(n+1)/2⟹1/1+2+..+n=2/n(n+1)
M=1/2(1 + 2/2.(2+1) + 2/3.(3+1)+...+2/2020.(2020+1))
M=1/2.(2/1.2 + 2/2.3 +2/3.4+..+2/2020.2021)
M=1/2.2.(1/1.2 + 1/2.3 +1/3.4 +..+1/2020.2021)
M=(1-1/2+1/2-1/3+1/3-1/4+...+1/2020-1/2021)
M=1-1/2021=2020/2021
TICK HAY NHẤT CHO MIK NHA^^, CẢM ƠN BẠN