The function \( f(x) = 2x^3 - 3x^2 - 12x + 4 \), \( x \in \mathbb{R} \) has:
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For a polynomial, find critical points by setting the first derivative to zero, then use the second derivative test to classify them as local maxima (\( f''(x)<0 \)) or minima (\( f''(x)>0 \)).