-Đặt `A = 1/5^2 + 1/6^2 +.......+ 1/100^2`
-Ta có: `1/5^2 > 1/5.6`
`1/6^2 > 1/5.6`
.........................
`1/100^2 > 1/100.101`
→ `1/5^2 + 1/6^2 +.......+ 1/100^2 > 1/5.6 + 1/6.7 +........+ 1/100.101`
→ `A > 1/5 - 1/6 + 1/6 - 1/7 +...........+ 1/100 - 1/101`
→ `A > 1/5 - 1/101`
→ `A > 96/505 > 96/576 = 1/6`
→ `A > 1/6`
Vậy `A > 1/6`