n[ln(n+2)-lnn]=nln(n+2)/n=nln(1+2/n)=2ln[(1+2/n)^(n/2)]
当n趋于无穷时(1+2/n)^(n/2)趋近于e
所以n[ln(n+2)-lnn]=2ln[(1+2/n)^(n/2)]趋近于2lne=2