Find the local maximum and minimum of $LaTeX: \displaystyle f(x) = - 3 x^{3} + 9 x + 2$ .

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = 9 - 9 x^{2}$ . Solving $LaTeX: \displaystyle 9 - 9 x^{2} = 0$ gives $LaTeX: \displaystyle x = \left[ -1, \ 1\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( -1 \right) = 18$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(-1\right) = -4$ is a local minimum.
$LaTeX: \displaystyle f''\left( 1 \right) = -18$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(1\right) = 8$ is a local maximum.