tailieunhanh - Calculus: An Integrated Approach to Functions and their Rates of Change, Preliminary Edition Part 89

Calculus: An Integrated Approach to Functions and their Rates of Change, Preliminary Edition Part 89. A major complaint of professors teaching calculus is that students don't have the appropriate background to work through the calculus course successfully. This text is targeted directly at this underprepared audience. This is a single-variable (2-semester) calculus text that incorporates a conceptual re-introduction to key precalculus ideas throughout the exposition as appropriate. This is the ideal resource for those schools dealing with poorly prepared students or for schools introducing a slower paced, integrated precalculus/calculus course | Computing Volumes 861 EXAMPLE Let s model a bagel by revolving a disk of radius 1 centered at the origin about the vertical line x 2. What is the volume of the bagel x 2 Figure SOLUTION We can slice the disk either along the x-axis or along the y-axis. The former will result in cylindrical shells and the latter in annuli or more appropriately bagel chips. We ll opt for slicing along the x-axis. We can revolve the half-disk in the first two quadrants obtaining the volume of the top half of the bagel and doubling it to get the final answer. Notice that if we revolved the half-disk in quadrants I and IV around the line x 2 we would get less than half the total volume. Why Partition -1 1 into n equal pieces using a standard partition. volume of the ith cylindrical shell 2nrihi Ax ri the distance between x 2 and xi 2 - xi Note that this holds regardless of the sign of xi. volume of i th cylindrical shell 2n 2 xi 1 x2 Ax volume of half of the bagel j 2n 2 x j 1 x2 dx 1 _ 2n 2x 1 x2 xV1 x2 dx The first integral we can recognize as giving the area of a semicircle of radius 1. 862 CHAPTER 28 More Applications of Integration 4n x2 dx 4n 1 7 2n 2n 2 The second integral has an integrand that is an odd function. x 1 x 2 x 1 x2 Therefore y x V1 - x2 dx 0 Volume of the whole bagel 2 2n2 4n2. Observation. Suppose this was not a real bagel but only a model made of clay. Suppose further that we chopped it open with a cleaver as shown in Figure and opened it up into a solid cylinder of radius 1. The length of the cylinder should be neither the outer circumference of the bagel nor the inner circumference of the bagel but the circumference corresponding to the dotted line shown. The volume of this cylinder is the area of the circle n times the length 4n giving 4n2. This is precisely the volume of the bagel. In other words as you unroll the clay there will be cracking on one side and buckling on the other and they exactly cancel out. Figure

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