Find the local maximum and minimum of $LaTeX: \displaystyle f(x) = 7 x^{3} + 11 x^{2} + x + 3$ .

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = 21 x^{2} + 22 x + 1$ . Solving $LaTeX: \displaystyle 21 x^{2} + 22 x + 1 = 0$ gives $LaTeX: \displaystyle x = \left[ -1, \ - \frac{1}{21}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( -1 \right) = -20$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(-1\right) = 6$ is a local maximum.
$LaTeX: \displaystyle f''\left( - \frac{1}{21} \right) = 20$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(- \frac{1}{21}\right) = \frac{3938}{1323}$ is a local minimum.