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Applications of Integration

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- In this chapter we explore some of the applications of the
- definite integral by using it for
- Computing the area between curves
- Computing the volumes of solids
- Computing the work done by a varying force
- Computing average value of a function
- The common theme is the following general method, which is similar to the one we used to find areas under curves:
- We break up a Q quantity into a large number of small parts. We next approximate each small part by a quantity of the form and thus approximate Q by a Riemann sum. Then we take the limit and express Q as an integral Finally we evaluate the integral using the Fundamental Theorem of Calculus or the Midpoint Rule.

We define the area of as the limiting value of the sum of the areas of the following approximating rectangles.

In trying to find the volume of a solid we face the same type of problem as in finding areas. We have an intuitive idea of what volume means, but we must make this idea precise by using calculus to give an exact definition of volume.

We start with a simple type of solid called a cylinder (or, more precisely, a right cylinder). As illustrated in Figure 1(a), a cylinder is bounded by a plane region , called the base, and a congruent region in a parallel plane. The cylinder consists of all points on line segments that are perpendicular to the base and join to . If the area of the base is and the height of the cylinder (the distance from to ) is , then the volume of the cylinder is defined as

In particular, if the base is a circle with radius , then the cylinder is a circular cylinder with volume [see Figure 1(b)], and if the base is a rectangle with length and width then the cylinder is a rectangular box (also called a rectangular parallelepiped) with volume [see Figure 1(c)].

The solids in Examples 1–5 are all called solids of revolution because they are obtained by revolving a region about a line. In general, we calculate the volume of a solid of revolution by using the basic defining formula

We find the cross-sectional area in one of the following ways:

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Review Problems

Find the volume of the solid obtained by rotating the region bounded by the given curves about the specified line. Sketch the region, the solid, and a typical disk or washer

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Set up, but do not evaluate, an integral for the volume of the solid obtained by rotating the region bounded by the given curves about the specified line.

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Use the method of cylindrical shells to find the volume generated by rotating the region bounded by the given curves about the Specific line. Sketch the region and a typical shell.

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Find the average value of the function on the given interval.

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A tank is full of water. Find the work required to pump the water out of the outlet.

A tank is full of water. Find the work required to pump the water out of the outlet. Use the fact that water weighs 62.5 lb/ft.