f(x+delta x)-f(x)/delta x solve for the solution
= [2(x+delta x)^2 -4 -2x^2 +4]/(delta x) = [2x^2 + 4(x delta x +(delta x)^2 -2x^2 +4]/(delta x) = 4x + delta x Which becomes 4x when delta x -> 0
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