Answer
a) If $f$ has a local maximum or minimum at $c$, and if $f'(c)$ exists, then $f'(c)=0$.
b) A critical number of a function $f$ is a number $c$ in the domain of $f$ such that either $f'(c)=0$ or $f'(c)$ does not exist.
Work Step by Step
The answers to both parts of the question are material to memorize.
