分子分母同趋于0可用洛必达法则 lim(x趋于-1)(sin(x+1))/2(x+1) = lim(x趋于-1)(sin(x+1))'/(2(x+1))' = lim(x趋于-1)(cos(x+1))/2 = cos0/2 = 1/2 如果没学过洛必达法则,可以用不等式逼近 x在一定范围内有 x/(x+1)
