令tanx=t,则x=arctant ,x→0时,t→0注意到arctant与t是等价的,所以有原式 = lim (tant-t)/(arctant)³=lim (tant-t)/t³ 《再用洛毕塔》=lim (sec²t-1)/3t² = lim tan²t/3t²= 1/3...