求极限时函数分子分母颠倒,为什么极限也分子分母颠倒?比如说依据什么知道的有lim sinx/x=1(x趋于0),就有lim x/sinx=1（x趋于0）?这个是依据什么定理得出的?

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• 求极限时函数分子分母颠倒,为什么极限也分子分母颠倒?比如说依据什么知道的有lim sinx/x=1(x趋于0),就有lim x/sinx=1（x趋于0）?这个是依据什么定理得出的?
• 关于有泰勒公式求极限的问题用泰勒公式来求：当x趋于0时lim(e^x * sinx - x(1+x))/(x^3)的极限我这样算对不对：分子=(e^x * sinx - x(1+x))=[1 + x + x^2/2 + o(x^2)][x + o(x)] - x(1+x) = x^3/2 + o(x^3)再加上分母得1/2,而参考答案则为.(e^x * sinx - x(1+x))=[1 + x + x^2/2 + o(x^2)][x - x^3/6 + o(x^3)] - x(1+x) = x^3/3 + o(x^3)最后得1/3答案的sinx比我多展开了,为什么要这样呢?而我的为什么不对?
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