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Critical Numbers of a Polynomial

APCALC-UKWBRN

What are the critical numbers for the following function?

$$f(x) = \cfrac{1}{3}x^{3}-x^{2}-8x$$

A

$x=4 \text{ and } x=\cfrac{1}{2}$

B

$x=-4 \text{ and } x=2$

C

$x=4 \text{ and } x=-2$

D

$x=\cfrac{1}{4} \text{ and } x=2$