Lời giải:
`A=1/2^2+1/3^2+...+1/2020^2`
`A<1/3^2+1/4^2+....+1/2020^2`
`A<1/(3.3)+1/(4.4)+....+1/(2020.2020)`
`A<1/(2.3)+1/(3.4)+1/(4.5)+...+1/(2019.2020)`
`A<1/2-1/3+1/3-1/4+...+1/2019-1/2020`
`A< 1/2-1/2020`
`A< 1009/2020 < 2019/2020`
Vậy `A<2019/2020`