Đáp án:
Giải thích các bước giải:
`A=1/2^2+1/4^2+1/6^2+...+1/(2n)^2`
`=>A=1/2.2+1/4.4+1/6.6+...+1/(2n.2n)`
`=>A<1/1.2+1/2.4+1/4.6+...+1/((2n-2).2n`
`=>A<1/2.(1+2/2.4+2/4.6+...+2/((2n-2).2n))`
`=>A<1/2.(1+1/2-1/4+1/4-1/6+...+1/(2n-2)-1/(2n))`
`=>A<1/2.(1-1/(2n))`
`=>A<1/2-1/(4n)<1/2`
`=>A<1/2`
Vậy `A<1/2`