when: f(x)=x^2+5x
= x^2 + 2xh + h^2 + 5x + 5h ( f(x+h) - f(x) )/h = (x^2 + 2xh + h^2 + 5x + 5h - x^2 - 5x) /h = (2xh + h^2 + 5h)/h = 2x + h + 5 , h ≠ 0
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