Finding Extrema on Intervals
Where do extrema occur? At critical numbers - points where either the derivative equals zero f′(c)=0 or the derivative doesn't exist. Think of these as places where the function flattens out horizontally or has a sharp corner.
To find the absolute extrema on a closed interval a,b, follow these steps:
- Find all critical numbers in the interval wheref′(x)=0orf′doesn′texist
- Calculate the function value at each critical number AND at the endpoints
- The largest value is the absolute maximum; the smallest is the absolute minimum
For example, if we wanted to find extrema for f(x) = 3x² - 4x³ on −1,2, we'd first find critical numbers by solving f'(x) = 0, giving us x = 0 and x = 1. Then we'd evaluate f(-1), f(0), f(1), and f(2), discovering that the absolute maximum is at (2, 16) and the absolute minimum at (1, -1).
Pro Tip: Not all critical points lead to extrema! In our example, x = 0 is a critical point but doesn't give us a maximum or minimum - it's what we call an inflection point where the curve changes direction.