1、x→∞时,1/(x+1)是无穷小,sin(x+1)是有界函数,所以结果是0
2、x→0时,ln(1+x^k) 等价于 x^k,x＋x^(1/3)＝x^(1/3)[1＋x^(2/3)],1＋x^(2/3)的极限为1,所以 x＋x^(1/3) 等价于 x^(1/3),所以 k＝1/3