-n/(n+1)=1/(n+1)-1
-(n+1)/(n+2)=1/(n+2)-1
∵n表示正整数
∴1/(n+1)＞1/(n+2)
∴1/(n+1)-1＞1/(n+2)-1
∴-n/(n+1)＞-(n+1)/(n+2)