Ta có:

`A = 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90`

`A = 1/(2.3) + 1/(3.4) + 1/(4.5) + 1/(5.6) + 1/(6.7) + 1/(7.8) + 1/(8.9) + 1/(9.10)`

`A = 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10`

`A = 1/2 - 1/10 `

`A = 5/10 - 1/10 `

`A = 4/10`

`A = 2/5 `

Vì `2/5 < 1` nên `A < 1`

Vậy `A < 1`