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Average Value

Given n numbers yl, ..., yn, their average value is defined as


If all the yi are replaced by the average value yave, the sum will be unchanged,

y1 + ... + yn = yave + ... + yave = nyave.

If f is a continuous function on a closed interval [a, b], what is meant by the average value of f between a and b (Figure 6.5.1)?


Figure 6.5.1

Let us try to imitate the procedure for finding the average of n numbers. Take an infinite hyperreal number H and divide the interval [a, b] into infinitesimal subintervals of length dx = (b - a)/H. Let us "sample" the value of f at the H points a, a + dx, a + 2 dx, ..., a + (H - 1) dx. Then the average value of f should be infinitely close to the sum of the values of f at a, a + dx, ..., a + (H - 1) dx, divided by H. Thus





and we have



Taking standard parts, we are led to


Let f be continuous on [a, b]. The average value of f between a and b is


Geometrically, the area under the curve y = f(x) is equal to the area under the constant curve y = fave between a and b,


Example 1: Finding Average Value

Last Update: 2006-11-25