Find the local maximum and minimum of $LaTeX: \displaystyle f(x) = 3 x^{3} + 15 x^{2} + 9 x + 2$ .

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = 9 x^{2} + 30 x + 9$ . Solving $LaTeX: \displaystyle 9 x^{2} + 30 x + 9 = 0$ gives $LaTeX: \displaystyle x = \left[ -3, \ - \frac{1}{3}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( -3 \right) = -24$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(-3\right) = 29$ is a local maximum.
$LaTeX: \displaystyle f''\left( - \frac{1}{3} \right) = 24$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(- \frac{1}{3}\right) = \frac{5}{9}$ is a local minimum.