Đáp án:
`A = (1 - 1/2)(1 - 1/4)(1 - 1/6)..................(1 - 1/195)`
` =1/2 . 3/4 . 5/6 ................ 194/195`
Mà `1/2 < 2/3 , 3/4 < 4/5 ,......,194/195<195/196`
`⇒ A < 2/3 . 4/5 ........ 195 / 196`
`⇒ A² < 1/2 . 2/3 . 3/4 . 4/5 ............. 194/195 . 195/196`
`A² < 1/196`
`⇒ A² < (1/14)^2`
`⇒ A < 1/14` (đpcm)