lim趋于0怎么运算

lim(x→0)(tanx-x)/(x^3)

=lim(x→0){[(secx)^2-1]/3x^2}

=lim(x→0)(tan^2)/(3x^2)

=lim(x→0)(x^2)/(3x^2)（等价无穷小tanx~x）

=1/3