The difference quotient is used to find the line of a tangent graph. We see a secant line when the curve is touched by a secant line on two points. In contrast, the tangent line only touches the curve at one particular point. A tangent line can be horizontal along the curve. The different points on the curve have different values these values are not numerical but they are substituted by [x+h, f(x+h)], [x, f(x))], and these are the coordinates which allow us to derive the difference quotient. We can say delta x instead of h because it basically means the same thing. It just depends on the units you are using to solve for the graph.
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