-n+1分之n -(-n+2分之n+1)
=-n/(n+1) +(n+1)/(n+2)
=((n+1)^2-n(n+2))/(n+1)(n+2)
=(n^2+2n+1-n^2-2n)/(n+1)(n+2)
=1/(n+1)(n+2) >0
所以-n+1分之n > -n+2分之n+1