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$$ \lim_{\Delta x \rightarrow 0}{\frac{\Delta y}{\Delta x}}-{\frac{A\Delta x}{\Delta x}}=0 $$

\lim_{\Delta x \rightarrow 0}{\frac{\Delta y}{\Delta x}}-{\frac{A\Delta x}{\Delta x}}=0

When $$a \ne 0$$ , there are two solutions to $$(ax^2 + bx + c = 0)$$ and they are $$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$

test $x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$