用等价无穷小代换
lim(x→0）tan2x/[(1+x+x^2)^1/2-1 ]
=lim(x→0）2x/[1/2(x+x^2) ]
=lim(x→0）2x/[1/2x(1+x) ]
=lim(x→0）2x/[1/2x ]
=4