Ta có: `3^2/10^2 = 3^2/10.10 > 3^2/10.11`
`3^2/11^2 = 3^2/11.11 > 3^2/11.12`
`...............................................`
`3^2/99^2 = 3^2/99.99 > 3^2/99.100`
`=> 3^2/10^2 + 3^2/11^2 +....+3^2/99^2 > 3^2/10.11 + 3^2/11.12 + ....+3^2/99.100`
`=> A > 3^2( 1/10.11 + 1/11.12 +...+1/99.100)`
`=> A > 3^2( 1/10- 1/11 + 1/11 - 1/12 +...+1/99-1/100)`
`=> A > 3^2( 1/10 - 1/100)`
`=> A > 9. 9/100`
`=> A > 81 /100`
Vì `81/100 > 4/5`
`=> A> 4/5`
Vậy `A> 4/5`
