Bài 17:
`1/(2000.1999)-1/(1999.1998)-...-1/(2.1)`
`=(1/1999-1/2000)-(1/1998-1/9999)-...-(1-1/2)`
`=1/1999-1/2000-1/1998+1/9999-...-1+1/2`
`=1/2000-1`
`=-2001/2000`
Bài 23:
`B∈Z`
`⇔6n+7\vdots2n+3`
`⇔6n+9-2\vdots2n+3`
`⇔-2\vdots2n+3`
`⇔2n+3∈Ư(2)∈{±1;±2}`
`⇔n∈{-1;-2;-1/2;-5/2}`
mà `n∈Z`
`⇔n∈{-1;-2}`
b) `B` Min
`⇔(6n+7)/(2n+3)` Min
`⇔3-2/(2n+3)` Min
`⇔2/(2n+3)` Max
`⇔2n+3` nguyên dương nhỏ nhất
`⇔2n+3=1`
`⇔2n=-2`
`⇔n=-1`