x趋于正无穷
分子分母都趋于0
于是用洛不答法则:
=lim(ln(1+2/x)'/(arccotx)'
=lim(1/(1+2/x)*-2/x^2)/[-1/(1+x^2)]
=lim(1+x^2)*2/(x^2*(1+2/x))
=lim2(1+x^2)/(x^2+2x)
=2