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Let f be defined at c. If f'(c) = 0 or f' is undefined at c, then c is a critical number of f.
Why is this important?
Because relative extrema occur only at critical numbers. The way to find the maximum and minimum of a function, f, on an interval [a,b] is to find the value of f at each critical number and at the two endpoints of the interval. The largest value obtained this way is the maximum of f, and the smallest is its minimum.
Relative Extrema Occur Only at Critical Numbers -- if c is an extremum then f'(c)=0 or f'(c) is undefined
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