A=1/11+1/12+1/13+…+1/70. Chứng minh A>4/3 Ai giúp em với ạ! Em cảm ơn!!! ❤ 07/08/2021 Bởi Arya A=1/11+1/12+1/13+…+1/70. Chứng minh A>4/3 Ai giúp em với ạ! Em cảm ơn!!! ❤
`A=1/11+1/12+1/13+…+1/70` `A>(1/20+1/20+…+1/20)+(1/30+1/30+…+1/30)+…+(1/70+..+1/70+1/70)` `A>1/2+1/3+1/4+1/5+1/6+1/7` `A>223/140` Mà `223/140=4/3` nên `A>223/140` hay `A>4/3` Bình luận
Đáp án: `A>4/3` Giải thích các bước giải: ta xét : `A=(1/11+1/12+…+1/20)+(1/21+1/22+…+1/30)+(1/31+1/32+…+1/60)+(1/61+1/62+…+1/70)` ta có : `=>` `1/11+1/12+…+1/20` `>` `1/20+1/20+…+1/20=10/20=1/2` `=>` `1/21+1/22+…+1/30` `>` `1/30+1/30+…+1/30=10/30=1/3` `=>` `1/31+1/32+…+1/60` `>` `1/60+1/60+…+1/60=30/60=1/2` `=>` `1/2+1/3+1/2=4/3` nên `A>4/3` Bình luận
`A=1/11+1/12+1/13+…+1/70`
`A>(1/20+1/20+…+1/20)+(1/30+1/30+…+1/30)+…+(1/70+..+1/70+1/70)`
`A>1/2+1/3+1/4+1/5+1/6+1/7`
`A>223/140`
Mà `223/140=4/3` nên `A>223/140` hay `A>4/3`
Đáp án:
`A>4/3`
Giải thích các bước giải:
ta xét :
`A=(1/11+1/12+…+1/20)+(1/21+1/22+…+1/30)+(1/31+1/32+…+1/60)+(1/61+1/62+…+1/70)`
ta có :
`=>` `1/11+1/12+…+1/20` `>` `1/20+1/20+…+1/20=10/20=1/2`
`=>` `1/21+1/22+…+1/30` `>` `1/30+1/30+…+1/30=10/30=1/3`
`=>` `1/31+1/32+…+1/60` `>` `1/60+1/60+…+1/60=30/60=1/2`
`=>` `1/2+1/3+1/2=4/3`
nên `A>4/3`