左极限就是函数从一个点的左侧无限靠近该点时所取到的极限值，且误差可以小到我们任意指定的程度，只需要变量从坐标充分靠近于该点。 函数在一点处极限存在时，函数在此处的左极限和右极限均存在，且左右极限相等。
因为我是一个坏人，所以我们班长左看右看横看竖看都觉的我不顺眼，没事总是要找找我的毛病，不过人的忍耐是有极限的。
Because I was a baddy, so our monitor was offensive to the eye with me all the time, and always nicked me, my patience was limited.
并讨论说明了导函数的右(左)极限与右(左)导数之间的关系。
And demonstrates the relations between the right (left) limit of function and right (left) derivative.
导数极限定理可以引出左(右)导数极限定理、区间端点导数极限定理。
The derivative limit theorem may draw out the left (right) derivative limit theorem and about the region end point.
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