I don't know how well you can see it in this figure here, is the slope of a straight line connecting these two points, and as the points come closer and closer, the straight line would become tangent to the curve.

我不清楚你们对于这个图像的理解,这条连接两点的斜线,随着两点距离逐渐缩小,两点连线会变成曲线的切线

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