使用函数极限的定义；

Use the limit definition of the function;

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从初等函数入手，以极限定义为基础，按照极限的定义来进行论证和阐述。

Proceed with elementary function, prove and explain promptly since defining for the foundation , according to the bottom of limit with limit.

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我们知道，极限是导数知识的基础，对于学习导数的定义具有重要的作用。

We know that the limit is the basis for derivative knowledge, for learning an important role in the definition of the derivative of.

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