5.5: Absolute Extrema
Recall that given a function f (x), the absolute (global) minimum is the smallest value of f on its
domain, and the absolute (global) maximum is the largest value of f on its domain.
In plural, extrema are the maximum and minimum of f .
The Extreme Value Theorem: If f is continuous on a closed interval [a, b], then f attains its absolute
maximum value f (c) and absolute minimum value f (d) at some number c and d in [a, b].
Note:
• An absolute extremum for a continuous function on a closed interval can be a relative extremum
or one of the end points of the interval.
• An extreme value can be taken on more than once.
• A function may not have an absolute maximum or minimum if it is not continuous or the interval
is not closed and bounded.
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Procedure for finding absolute extrema: Let f be a continuous function on a closed and bounded interval
[a, b].
1) Locate all critical values on (a, b).
2) Evaluate f (x) at those the critical values and also at a and b.
3) The absolute maximum of f (x) on [a, b] will be the largest number found in Step 2, while the absolute
minimum of f (x) on [a, b] will be the smallest number found in Step 2.
Example 1. Locate the value(s) where each function attains an absolute maximum and the value(s)
where the function attains an absolute minimum, if they exist, of the given function on the given interval.
a) f (x) = −x2 + 2x + 1 on [−1, 4].
b) f (x) = x3 + x2 − 4x + 1 on [−2, 2].
c) f (x) =
1
on (−∞, 6].
x
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5.6: Optimization and Modeling
Six-Step Mathematical Modeling for Optimization:
1. Identify the variables (unknown quantities) and denote each with a symbol.
2. Identify the variable that is to be maximized or minimized and express it as a function of the other
variables.
3. Find any relationships between the variables.
4. Express the quantity to be maximized or minimized as a function of only one variable. To do this,
you might need to solve for one of the variables in one of the equations in Step 3 and substitute this
into the formula found in Step 2.
5. Determine the interval over which the function is to be minimized or maximized.
6. Steps 1 through 5 leave you with a function of a single variable on a certain interval to be maximized
or minimized. Proceed now as in previous sections.
Example 2. Find two positive number whose product is 169 and whose sum is a minimum.
Example 3. Find two number whose sum is 80 and whose product is maximum.
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Example 4. A farmer with 1000 ft of fencing wants to enclose a rectangular area. What is the largest
possible area?
Example 5. A box with a square base and open top must have a volume of 500,000 cm3 . Find the
dimensions of the box that minimize the amount of material used.
Example 6. A box with an open top is to be constructed from a square piece of cardboard, 5 ft wide,
by cutting out a square from each of the four corners and bending up the sides. Find the largest volume
that such a box can have.
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Business Applications:
Recall that if C(x), the cost function, then the marginal cost function is the derivative C 0 (x) of the
cost function.
Let p(x) be the price-demand function. Recall that if x units are sold and the price per unit is p(x),
then the total revenue is R(x) = xp(x). The derivative R0 of the revenue function is the marginal
revenue function, i.e. the rate of change of revenue with respect to the number of units sold.
If x units are sold, then the total profit is P (x) = R(x) − C(x) and the marginal profit function P 0
is the derivative of the profit function. In order to maximize profit, we look for the critical value of
function P by solving:
P 0 (x) = R0 (x) − C 0 (x) = p(x) + xp0 (x) − C 0 (x) = 0.
The profit is maximized at the solution of that equation.
Example 7. Determine the production level that will maximize the profit for a company with cost and
demand functions C(x) = 1200 + 25x − 0.0001x2 and p(x) = 50 − 0.001x.
Example 8. Let the cost for a certain product be given by C(x) = 0.001x3 − 0.3x2 + 6x + 1000. Find
the production level at which the marginal cost function starts to increase.
Example 9. The manager of a 200-unit apartment complex knows from experience that all units will
be occupied if the rent is $500 per month. A market survey suggests that, on the average, one additional
unit will remain vacant for each $5 increase in rent. What rent should the manager charge to maximize
revenue?
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