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[raven ingot]

This is the definition of a derivative. What it’s explaining is that if you have f(x) = x^2, and you find the slope between the two points (x,x^2) and (x-h,(x-h)^2), you get the equation on the second line. That can be simplified (which was done in the third and fourth line), and the fifth line divides by h to get 2x + h. Since the limit of h approaches 0, that means the equation approaches 2x. That is why the derivative of x^2 is equal to 2x.