MATH 131-505 Spring 2015
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Wen
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4.2
4.2 Maximum and Minimum Values
Definition:
• Let c be a number in the domain D of a function f . Then f (c) is the
– absolute maximum value of f on D if f (c) ≥ f (x) for all x ∈ D.
– absolute minimum value of f on D if f (c) ≤ f (x) for all x ∈ D.
• An absolute maximum or minimum is sometimes called a global maximum or minimum.
• The maximum and minimum values of f are called extreme values of f .
• The number f (c) is a
– local maximum value of f if f (c) ≥ f (x) when x is near c.
– local minimum value of f if f (c) ≤ f (x) when x is near c.
Examples:
1. A function with infinitely many extreme values.
2. A function with no maximum or minimum
3. A function with a minimum value but no
maximum value
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MATH 131-505 Spring 2015
4.2
c
Wen
Liu
We have seen that some functions have extreme values, whereas others do not. The following theorem
gives conditions under which a function is guaranteed to possess extreme values.
The Extreme Value Theorem If f is continuous on a closed interval [a, b], then f attains an
absolute maximum value f (c) and an absolute minimum value f (d) at some numbers c and d in [a, b].
The last two figures on previous page show that a function need not possess extreme values if either
hypothesis (continuity or closed interval) is omitted from the Extreme Value Theorem.
Definition: A critical number of a function f is a number c in the domain of f such that either
f 0 (c) = 0 or f 0 (c) does not exist.
Examples:
4. (p. 266) Find the critical numbers of f (x) = x3/5 (4 − x).
5. Find the critical numbers of g(t) = |5t − 2|.
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MATH 131-505 Spring 2015
4.2
c
Wen
Liu
The Closed Interval Method To find the absolute maximum and minimum values of a continuous
function f on a closed interval [a, b]:
(1) Find the values of f at the critical numbers of f in (a, b).
(2) Find the values of f at the endpoints of the interval.
(3) The largest of the values from Steps 1 and 2 is the absolute maximum value; the smallest of these
values is the absolute minimum value.
Examples:
6. Find the absolute maximum and absolute minimum values of f (x) = x − ln(2x) on [0.5, 2].
7. Find the absolute maximum and absolute minimum values of g(x) = x3 − 6x2 + 9x + 9 on [−1, 5].
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