Bài 1 : A. 1/8×9 + 1/9×10 + 1/10×11 + 1/11×12 + ….. + 1/50×51 B . 1/9×11 + 1/11×13 + 1/13×15 + ….. + 2/97×99

`text{Đáp án:}`

`A=43/408`

`B=10/198`

`text{Giải thích các bước giải:}`

`a)`
`1/(8×9) + 1/(9×10) + 1/(10×11) + 1/(11×12) + ….. + 1/(50×51)`
`A=1/8-1/9+1/9-1/10+1/10-1/11+1/11-1/12+…+1/50-1/51`

`A=1/8-1/51`
`A=51/408-8/408`
`A=43/408`
`b)`
`2B=1/(9×11) + 1/(11×13) + 1/(13×15) + ….. + 1/(97×99)`
`2B=2/(9×11) + 2/(11×13) + 2/(13×15) + ….. + 2/(97×99)`
`2B=1/9-1/11+1/11-1/13+1/13-1/15+…+1/97-1/99`
`2B=1/9-1/99`
`2B=99/891-9/891`
`2B=90/891`
`B=90/891:2`
`B=90/891. 1/2`
`B=90/1782`
`B=10/198`