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For which real value of $x$ is $f(x)$ the greatest if:

$$ f(x) = ax^2 + bx + c $$

...(where $a < 0$) the greatest?

$ -\cfrac{b}{2a} $

$ -\cfrac{b}{a} $

$ \sqrt { { b }^{ 2 }-4ac } $

$ b^2-4ac $

$ \cfrac { -b\pm \sqrt { b^{ 2 }-4ac } }{ 2a } $