August 2003:

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August 2003:
A trapezoid has vertices (1,0), (4,4), (9,4) and (12,0). Calculate its volume when it is revolved about the x-axis. Show the methods that you use. Now the trapezoid is rotated about the y-axis. Again calculate the volume generated and show all the methods you use. Then find the ratio of the volumes.

First, the legs of the trapezoid are given linear equations. Using the the coordinates, we obtain y = 4/3*x - 4/3 for one and y = -4/3*x + 16 for the other. In terms of x, they are x = 3y/4 + 1 and x = -3y/4 + 12. The volume of the trapezoid's revolution about the x-axis can be obtained by integrating pi*r^2 for the two lines and the horizontal line y = 4, where r is the equation in terms of x. Adding up the integrals yields to 112pi. For the revolution about the y-axis, the washer formula is used, which is pi*R^2 - pi*r^2 where R is the equation of the right leg in terms of y and r is the equation of the left leg in terms of y. Integraion leads to 416pi. The ratio of the two volumes is about 3.7.


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