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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