The volume X of the region of points ( x , y , z ) such that ( x 2 + y 2 + z 2 + 8 ) 2 ≤ 3 6 ( x 2 + y 2 ) .
Find [ X ] .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Let's take a cylindrical coordinates approach here. Let x 2 + y 2 = r 2 which gives us:
( r 2 + z 2 + 8 ) 2 ≤ 3 6 r 2 ⇒ r 2 + z 2 + 8 ≤ 6 r ⇒ ( r 2 − 6 r + 9 ) + z 2 − 1 ≤ 0 ⇒ z 2 ≤ 1 − ( r − 3 ) 2 ;
or − 1 − ( r − 3 ) 2 ≤ z ≤ 1 − ( r − 3 ) 2 . At z = 0 we obtain the following x y − p l a n e curves: r 2 + 8 ≤ 6 r ⇒ r 2 − 6 r + 8 ≤ 0 ⇒ ( r − 2 ) ( r − 4 ) ≤ 0 ⇒ 2 ≤ r ≤ 4 which sweep an entire circle ( 0 ≤ θ ≤ 2 π ). 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 ≈ 5 9 . 2 1 7 6
or ⌊ 5 9 . 2 1 7 6 ⌋ = 5 9 .
Problem Loading...
Note Loading...
Set Loading...
It's a torus with major radius of 3 and minor radius of 1 This can be seen by letting y = 0 and solving for z to get
z = 1 − ( x ± 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 = 5 9 . 2 1 7 6 . . .