lm hộ mình 1 câu nhá

`3 . 1/1.2 - 5 . 1/2.3 + 7 . 1/3.4 - ... + 15 . 1/7.8 - 17 . 1/8.9`

`= 3/(1.2)-5/(2.3)+7/(3.4)-...+15/(7.8)-17/(8.9)`

`= (1/1+1/2)-(1/2+1/3)+(1/3+1/4)-...+(1/7+1/8)-(1/8+1/9)`

`= 1/1+1/2-1/2-1/3+1/3+1/4-...+1/7+1/8-1/8-1/9`

`=1 - 1/9`

`=8/9`.