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Question

Determine the value of bb so that the volume of the ellipsoid

x2+y2b2+z29=1x^2+\dfrac{y^2}{b^2}+\dfrac{z^2}{9}=1

is 16π16 \pi.

Solution

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Answered 2 years ago
Answered 2 years ago
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One may be tempted to solve this via integration, but the task does not require us to do so. Instead, we can use the fact that the formula for the volume of the ellipsoid is:

V=4π3abcV=\dfrac{4\pi}{3}abc

where the ellipsoid is given with the equation:

x2a2+y2b2+z2c2=1\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}=1

The equation we are given is:

x21+y2b2+z29=1\dfrac{x^2}{1}+\dfrac{y^2}{b^2}+\dfrac{z^2}{9}=1

From the equation we read a=1a=1 and c=3c=3. The volume has to be 16π16\pi. Finally we have:

4π31b3=16π\dfrac{4\pi}{3}\cdot 1\cdot b\cdot 3=16\pi

Simplify the left side to get:

4πb=16πb=44\pi b=16\pi\Rightarrow b=4

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