Cut Ellipsoid Volume

Calculus Level 5

An ellipsoid is given by

x 2 100 + y 2 225 + z 2 900 = 1. \dfrac{x^2}{100} + \dfrac{y^2}{225} + \dfrac{z^2}{900} = 1.

It is cut by the plane

x + 3 y + 2 z = 40. x + 3 y + 2 z = 40.

Find the volume of the region that is inside the ellipsoid and below the plane. It is assumed that the upward direction is along the positive z z -axis. This 3D region is depicted in the figure above.

If the volume is V V , enter V \lfloor V \rfloor as your answer.

Hint : Use scaling to transform the ellipsoid and the cutting plane into a sphere with a corresponding cutting plane.


The answer is 16202.

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1 solution

Hosam Hajjir
Aug 20, 2016

We start by transforming the given ellipsoid into the unit sphere (radius 1)

Define x = x / 10 , y = y / 15 , z = z / 30 x' = x / 10, y' = y / 15, z' = z / 30 , then

x 2 + y 2 + z 2 = 1 x'^2 + y'^2 + z'^2 = 1

The corresponding cutting plane, is derived by substituting x, y, z into its equation

( 10 x ) + 3 ( 15 y ) + 2 ( 30 z ) = 40 (10 x') + 3(15 y') + 2(30 z') = 40

or

10 x + 45 y + 60 z = 40 10 x' + 45 y' + 60 z' = 40

which simplifies to

2 x + 9 y + 12 z = 8 2 x' + 9 y' + 12 z' = 8

This plane cuts through the sphere. The distance from the center of the sphere is

a = 8 2 2 + 9 2 + 1 2 2 = 8 229 a = \dfrac{8}{\sqrt{2^2 + 9^2 + 12^2}} = \dfrac{8}{\sqrt{229}}

Now we use the following formula for calculating the volume of a cut sphere of radius R, with

a plane that is a distance a a away from the center.

V S = π { 2 3 R 3 + a R 2 a 3 3 } V_S = \pi \left \{ \dfrac{2}{3} R^3 + a R^2 - \dfrac{a^3}{3} \right \}

Substituting the calculated a a , and R = 1 R = 1 , results in

V S = 1.14607276 π V_S = 1.14607276 \pi

Finally we scale back this volume, to get the volume of the cut ellipsoid

V = ( 10 ) ( 15 ) ( 30 ) V S = 16202.222 V = (10)(15)(30) V_S = 16202.222 , therefore the answer is 16202.222 = 16202 \lfloor 16202.222 \rfloor = \boxed{16202} .

Good Calc III problem, Hosam!

tom engelsman - 4 years, 9 months ago

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