My Math Forum Partial Derivative Application for 3D object

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 March 21st, 2017, 07:54 PM #1 Senior Member   Joined: Jan 2017 From: Toronto Posts: 209 Thanks: 3 Partial Derivative Application for 3D object Find the volume of the largest rectangular box with edges parallel to the axes that can be inscribed in the ellipsoid 2x^2 + 72y^2 + 18z^2 = 288. Answer: 256/sqrt(3) I would be much appreciated if anyone kindly point me to the right direction. Thanks!
 March 21st, 2017, 08:33 PM #2 Senior Member     Joined: Sep 2015 From: USA Posts: 2,648 Thanks: 1476 well by symmetry your corner points will be $(\pm a, \pm b, \pm c)$ with volume $V=8abc$ you can reduce this a bit using the formula for the ellipsoid to get points at $\left(\pm a, \pm b, \pm \sqrt{\dfrac{288 - 2a^2 - 72b^2}{18}}\right)$ with volume $V=8ab \sqrt{\dfrac{288 - 2a^2 - 72b^2}{18}}$ As usual take the gradient of this expression, with respect to $a,~b$ and set it equal to the vector 0 and solve for $a,~b$ Solving this might be a bit of a mess but Mathematica returns a pretty digestible answer so it might not be that bad. The way to ensure that this extreme points is a maximum is a bit more complicated for a 3D than a 2D case. Do you know how to use the two dimensional 2nd derivative test to do this? Last edited by romsek; March 21st, 2017 at 09:13 PM.

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