Volume of region bounded by points satisfying the given inequality !

Calculus Level 5

The volume X X of the region of points ( x , y , z ) (x,y,z) such that ( x 2 + y 2 + z 2 + 8 ) 2 36 ( x 2 + y 2 ) . \left(x^{2}+y^{2}+z^{2}+8\right)^{2}\le 36\left(x^{2}+y^{2}\right).

Find [ X ] . [X].


The answer is 59.

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2 solutions

Michael Mendrin
Sep 18, 2014

It's a torus with major radius of 3 3 and minor radius of 1 1 This can be seen by letting y = 0 y=0 and solving for z z to get

z = 1 ( x ± 3 ) 2 z=\sqrt { 1-{ (x\pm 3) }^{ 2 } }

Symmetry takes care of the rest.

The volume of a torus is the area of the circular cross section multiplied by the length of the circumference of the circular centerline of the center of the circular cross section, or

( 2 π R ) ( π r 2 ) = 2 π 2 ( 3 ) ( 1 2 ) = 6 π 2 = 59.2176... \left( 2\pi R \right) \left( \pi { r }^{ 2 } \right) =2{ \pi }^{ 2 }(3)({ 1 }^{ 2 })=6{ \pi }^{ 2 }=59.2176...

Tom Engelsman
Feb 25, 2021

Let's take a cylindrical coordinates approach here. Let x 2 + y 2 = r 2 x^2+y^2=r^2 which gives us:

( r 2 + z 2 + 8 ) 2 36 r 2 r 2 + z 2 + 8 6 r ( r 2 6 r + 9 ) + z 2 1 0 z 2 1 ( r 3 ) 2 ; (r^2+z^2+8)^2 \le 36r^2 \Rightarrow r^2+z^2+8 \le 6r \Rightarrow (r^2-6r+9)+z^2 -1 \le 0 \Rightarrow z^2 \le 1 - (r-3)^2;

or 1 ( r 3 ) 2 z 1 ( r 3 ) 2 -\sqrt{1-(r-3)^2} \le z \le \sqrt{1-(r-3)^2} . At z = 0 z=0 we obtain the following x y p l a n e xy-plane curves: r 2 + 8 6 r r 2 6 r + 8 0 ( r 2 ) ( r 4 ) 0 2 r 4 r^2 + 8 \le 6r \Rightarrow r^2-6r+8 \le 0 \Rightarrow (r-2)(r-4) \le 0 \Rightarrow 2 \le r \le 4 which sweep an entire circle ( 0 θ 2 π 0 \le \theta \le 2\pi ). These parts finally converge into the following triple integral:

X = 0 2 π 2 4 1 ( r 3 ) 2 1 ( r 3 ) 2 r d z d r d θ = 6 π 2 59.2176 X = \int_{0}^{2\pi} \int_{2}^{4} \int_{-\sqrt{1-(r-3)^2}}^{\sqrt{1-(r-3)^2}} r dz dr d\theta = 6\pi^{2} \approx 59.2176

or 59.2176 = 59 . \lfloor 59.2176 \rfloor = \boxed{59}.

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