Đáp án:
Đặt `A =1 + 1/2 + 1/3 + ......... + 1/31 + 1/32`
`=( 1+ 1/2 + 1/3 + ....... + 1/6) + (1/7 + 1/8 + 1/9+ .... +1/12) + ...... + (1/25 + 1/26 + 1/27 + 1/28) + (1/29 + 1/30 + 1/31 + 1/32)`
Vì ` 1/7 > 1/12 ; 1/8 > 1/12 ; 1/9 > 1/12; 1/10>1/12 ; 1/11>1/12`
`⇒ 1/7 +1/8 + ... + 1/12>1/12 + 1/12 +... +1/12`
......................................................................................................
* `1/25 > 1/28 ; 1/26>128 ; 1/27>1/28`
`⇒ 1/25 + 1/26 + 1/27 + 1/28>1/28+1/28+ 1/28 + 1/28`
* `1/29>1/32; 1/30>1/32 ; 1/31> 1/32`
`⇒ 1/29 + 1/30 + 1/31+1/32> 1/32 + 1/32 + 1/32 + 1/32`
`⇔A> (1+1/2 + 1/3 + .... +1/6) + (1/12 + 1/12 + ....+1/12)+....... + (1/28 + 1/28+1/28+ 1/28) + (1/32+1/32 + 1/32+1/32)`
` = 49/20 + 1/2 + 1/3 + 1/4 + 1/7 + 1/8`
` = 3193/840>2940/840=7/2`
`⇔A>7/2`
`text{ XIN HAY NHẤT NHA}`
@toanisthebest
