limx趋向于无穷n2(arctan(a/n)-arctan[a/(n+1)]求极限!

n→∞,
limn²[arctan(a/n)－arctan(a/(n+1))]
＝limn²[arctan(a/n－a/(n+1))/
(1+a²/n(n+1))]
＝limn²arctan[a/(n(n+1)+a²)]
＝liman²/[a²+n(n+1)](等价无穷小量)
＝a- - 看不懂能一步步 解释下么公式的套用