We have seen that the area of the region under the graph of a
positive function f (x) on [a, b] is given by the definite
integral
f (x) dx. The above properties
may help extend this result. Indeed, consider the region
bounded above by the graph of f (x) and below by the
graph of g(x), and bounded on the sides by the vertical lines
x = a and x = b, with a
b. Of course, we assume that
g(x)
f (x) for
x
[a, b]. Then we have
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Remark. Sometimes you may be asked to find the area of the region bounded by the graph of two or more functions. The idea is to split the region into subregions of the type described above and then use the formula for the subregions.
Example. Let us find the area of the region
bounded by the graphs of
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Notice that the region is bounded above by g(x) and below by f (x). To find the boundary points which will give the vertical side lines we need to solve
Example. Find the area of the region bounded by
the three lines
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Before we proceed with any integration we need to find the points of intersection of the three lines represented by the functions
The region may be subdivided into two regions
bounded above by the graph of g(x) and below by h(x) with
0
x
, and
bounded
above by the graph of f (x) and below by h(x) with
x
2.
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So