The Need for Tangent to a curve at a point and its definition

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I was understanding derivative function when I thought that why "concept of tangent", was invented.If it was so because of influence of Physics - instantaneous velocity and other stuff then why tangent line is best fit to get the rate of change of y versus x ,and most importantly "how it would be defined in this sense"**I am following **Michael Spivak's book he has defined it slope as limit of slope of secant lines but would that limit always equal slope of tangent line when we define it in this way.please help!!