Ta thấy :
`1/2^2 < 1- 1/2`
`1/3^2 < 1/2 - 1/3`
`1/4^2 > 1/3 - 1/4`
` . . . `
`1/10^2 < 1/9 - 1/10`
`=>A = 1/2^2 + 1/3^2 + 1/4^2 + ... + 1/10^2 < 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4+ ... + 1/9 - 1/10`
`=>A = 1/2^2 + 1/3^2+ 1/4^2 + ... + 1/10^2 < 1 - 1/10 < 3/4 `
`=> A = 1/2^2 + 1/3^2 + 1/4^2 + ... +1/10^2 < 3/4`