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Find the absolute maximum and absolute minimum of f ( x ) = x 2 4 x + 3 over the interval [ 1 , 4 ] .

The absolute maximum is 3 and it occurs at x = 4 . The absolute minimum is −1 and it occurs at x = 2 .

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At this point, we know how to locate absolute extrema for continuous functions over closed intervals. We have also defined local extrema and determined that if a function f has a local extremum at a point c , then c must be a critical point of f . However, c being a critical point is not a sufficient condition for f to have a local extremum at c . Later in this chapter, we show how to determine whether a function actually has a local extremum at a critical point. First, however, we need to introduce the Mean Value Theorem, which will help as we analyze the behavior of the graph of a function.

Key concepts

  • A function may have both an absolute maximum and an absolute minimum, have just one absolute extremum, or have no absolute maximum or absolute minimum.
  • If a function has a local extremum, the point at which it occurs must be a critical point. However, a function need not have a local extremum at a critical point.
  • A continuous function over a closed, bounded interval has an absolute maximum and an absolute minimum. Each extremum occurs at a critical point or an endpoint.

In precalculus, you learned a formula for the position of the maximum or minimum of a quadratic equation y = a x 2 + b x + c , which was m = b ( 2 a ) . Prove this formula using calculus.

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If you are finding an absolute minimum over an interval [ a , b ] , why do you need to check the endpoints? Draw a graph that supports your hypothesis.

Answers may vary

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If you are examining a function over an interval ( a , b ) , for a and b finite, is it possible not to have an absolute maximum or absolute minimum?

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When you are checking for critical points, explain why you also need to determine points where f ( x ) is undefined. Draw a graph to support your explanation.

Answers will vary

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Can you have a finite absolute maximum for y = a x 2 + b x + c over ( , ) ? Explain why or why not using graphical arguments.

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Can you have a finite absolute maximum for y = a x 3 + b x 2 + c x + d over ( , ) assuming a is non-zero? Explain why or why not using graphical arguments.

No; answers will vary

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Let m be the number of local minima and M be the number of local maxima. Can you create a function where M > m + 2 ? Draw a graph to support your explanation.

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Is it possible to have more than one absolute maximum? Use a graphical argument to prove your hypothesis.

Since the absolute maximum is the function (output) value rather than the x value, the answer is no; answers will vary

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Is it possible to have no absolute minimum or maximum for a function? If so, construct such a function. If not, explain why this is not possible.

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[T] Graph the function y = e a x . For which values of a , on any infinite domain, will you have an absolute minimum and absolute maximum?

When a = 0

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For the following exercises, determine where the local and absolute maxima and minima occur on the graph given. Assume domains are closed intervals unless otherwise specified.

The function graphed starts at (−2.2, 10), decreases rapidly to (−2, −11), increases to (−1, 5) before decreasing slowly to (1, 3), at which point it increases to (2, 7), and then decreases to (3, −20).

Absolute minimum at 3; Absolute maximum at −2.2; local minima at −2, 1; local maxima at −1, 2

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Practice Key Terms 9

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Source:  OpenStax, Calculus volume 1. OpenStax CNX. Feb 05, 2016 Download for free at http://cnx.org/content/col11964/1.2
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