Đáp án:
Vì:
`1/(2^2) < 1/(1 . 2)`
`1/(3^2) < 1/(2 . 3)`
...........................................
`1/(10^2) < 1/( 9 . 10)`
`⇒ D < 1/(1.2) + 1/(2.3) + ........... + 1/(9.10)`
` D < 1 - 1/2 + 1/2 - 1/3 + .......... + 1/9 - 1/10 = 1 - 1/10 < 1`
`⇒ D < 1` (đpcm)
`text{ Mathvietnam2k8}`