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

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = - 21 x^{2} - 4 x + 1$ . Solving $LaTeX: \displaystyle - 21 x^{2} - 4 x + 1 = 0$ gives $LaTeX: \displaystyle x = \left[ - \frac{1}{3}, \ \frac{1}{7}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( - \frac{1}{3} \right) = 10$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(- \frac{1}{3}\right) = \frac{73}{27}$ is a local minimum.
$LaTeX: \displaystyle f''\left( \frac{1}{7} \right) = -10$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(\frac{1}{7}\right) = \frac{151}{49}$ is a local maximum.