Helppppppppppppppppppppp

`color{#00a298ff} {bnk24}`

Ta có :

`1/2^2=1/(2.2)<1/(1.2)`

`1/3^2=1/(3.3)<1/(2.3)`

`1/4^2=1/(4.4)<1/(3.4)`

`..........................`

`1/9^2=1/(9.9)<1/(8.9)`

`->bbA<1/(1.2)+1/(2.3)+1/(3.4)+...+1/(8.9)`

`->bbA<1-1/2+1/2-1/3+1/3-1/4+...+1/8-1/9`

`->bbA1-1/9`

`->bbA<8/9` `(1)`

Ta lại có : 

`1/2^2=1/(2.2)>1/(2.3)`

`1/3^2=1/(3.3)>1/(3.4)`

`1/4^2=1/(4.4)>1/(4.5)`

`..........................`

`1/9^2=1/(9.9)>1/(9.10)`

`->bbA>1/(2.3)+1/(3.4)+1/(4.5)+...+1/(9.10)`

`->bbA>1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10`

`->bbA>1/2-1/10`

`->bbA>5/10-1/10`

`->bbA>4/10=2/5` 

`->bbA>2/5` `(2)`

Từ `(1)` và `(2)` ta thấy `8/9>bbA>2/5`

Vậy `8/9>bbA>2/5`