x→+∞时,arctanx->PI/2,x→-∞时,arctanx->-PI/2,lim(x->0)arctanx = lim(x->0)x = 0.lim(x->0)arctanx/x = lim(x->0)[1/(1+x^2)]/1 = 1所以,x→0时,arctanx和x是等价无穷小量.x→0时,arctanx ~ x....