Find the local maximum and minimum of $LaTeX: \displaystyle f(x) = x^{3} + 8 x^{2} + 5 x + 3$ .

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = 3 x^{2} + 16 x + 5$ . Solving $LaTeX: \displaystyle 3 x^{2} + 16 x + 5 = 0$ gives $LaTeX: \displaystyle x = \left[ -5, \ - \frac{1}{3}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( -5 \right) = -14$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(-5\right) = 53$ is a local maximum.
$LaTeX: \displaystyle f''\left( - \frac{1}{3} \right) = 14$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(- \frac{1}{3}\right) = \frac{59}{27}$ is a local minimum.