Such a Bullet

Calculus Level 3

The volume generated by rotating the ellipse x 2 16 + y 2 9 = 1 \dfrac{x^2}{16}+\dfrac{y^2}{9}=1 over the x x -axis can be written as a π a\pi . Find the value of a a .


The answer is 48.

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2 solutions

The volume can be written as two times the volume of a half (for positive values of x x ). This means that the volume equals the sum of all cylinders with height d x dx and base π y 2 ( x ) \pi\cdot y^2(x) . So the volume becomes: V = 2 0 4 π 9 ( 1 x 2 16 ) d x = 18 π 0 4 ( 1 x 2 16 ) d x V=2\displaystyle \int_{0}^{4}\pi\cdot 9(1-\frac{x^2}{16})dx=18\pi \displaystyle \int_{0}^{4} (1-\frac{x^2}{16})dx V = 18 π 4 18 π 1 16 4 3 3 = 48 π V=18\pi\cdot 4 - 18\pi\cdot \frac{1}{16}\cdot\frac{4^3}{3}=48\pi

Otto Bretscher
Apr 3, 2016

The semi-axes of the ellipsoid will be 4,3, and 3, and the volume is V = 4 × 3 × 3 × 4 π 3 = 48 π V=4\times3\times3\times\frac{4\pi}{3}=\boxed{48}\pi : Consider a unit sphere and stretch it out in the directions of the coordinate axes.

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