=[1-2+3-4+.+(2n-1)-2n]/(n-1)
=[1+3+5+7+……+（2n-1）]-[2+4+6+8+……+2n]
={（1+2n-1)×【（2n-1-1）/2+1】}/2-{（2+2n）×【（2n-2）/2+1】}/2
=n²-n-n²
=-n