Compute the volume of the solid enclosed by: 1. x/a+y/b+z/c=1, x=0, y=0, z=0. 2. x^2+y^2-2ax=0, z=0, x^2+y^2=z^2.

Passafaromx

Passafaromx

Answered question

2022-08-12

Volume of Solid Enclosed by an Equation
I'm having problems finding the triple integrals of equations. I guess it has to do with the geometry. Can someone solve the two questions below elaborately such that I can comprehend this triple integral thing once and for all:
Compute the volume of the solid enclosed by
1. x a + y b + z c = 1 , x = 0 , y = o , z = 0
2. x 2 + y 2 2 a x = 0 , z = 0 , x 2 + y 2 = z 2

Answer & Explanation

Macie Melton

Macie Melton

Beginner2022-08-13Added 19 answers

Step 1
The first equation is equivalent to
( x a ) 2 + y 2 = a 2 ,, which is a cylinder centered at (a,0,0) parallel to the z-axis. It's intersection with the xy-plane is a circle; the volume we want is the solid above this circle and below z 2 = x 2 + y 2 . Let the region bounded by this circle be C, so the volume is C x 2 + y 2 d A = 2 0 π 2 0 2 a cos θ r 2 d r d θ ,, where we used polar coordinates, in which r 2 = x 2 + y 2 and d A = r d r d θ (loosely speaking). The bounds come from the fact that half of the region C is also described by the polar curve r = 2 a cos θ, where 0 θ π 2 , and the multiplication by 2 accounts for the other half. Then, evaluating this, we get
2 0 π 2 [ r 3 3 ] 0 2 a cos θ d θ = 16 a 3 3 0 π 2 cos 3 θ d θ .
Step 2
This last integral can be evaluated using standard techniques, and turns out to be 2 3 for a final answer of 32 a 3 9

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