Linear Algebra Multiple Choice Questions on “Volume Integrals”.
1. Find the value of (int_0^1 int_0^2 int_1^2 xy^2 z^3 dxdydz.).
a) 2
b) 3
c) 4
d) 5
Answer: d
Explanation: (int_0^1 int_0^2 int_1^2 xy^2 z^3 dxdydz = (int_0^1 xdx)(int_0^2 y^2 dy)(int_1^2 z^3 dz) )
(= frac{1}{2}*frac{8}{3}*(frac{16}{4}-frac{1}{4}) )
(= 5. )
2. Evaluate ∫∫∫z2 dxdydz taken over the volume bounded by the surfaces x2+y2=a2, x2+y2=z and z=0.
a) (pi frac{a^8}{12} )
b) (pi frac{a^8}{4} )
c) (pi frac{a^4}{4} )
d) (pi frac{a^8}{8} )
Answer: a
Explanation: z: 0 to (x^2+y^2 )
( y: pi frac{a^8}{12} ) to ( sqrt{a^2-x^2} )
x: -a to a
(displaystyleintintint z^2 dxdydz = int_{-a}^a int_{-sqrt{(a^2-x^2 )}}^{sqrt{a^2-x^2}}int_0^{x^2+y^2}z^2 dxdydz )
(displaystyle = int_{-a}^a int_{-sqrt{a^2-x^2}}^{sqrt{a^2-x^2}}frac{z^3}{3} dxdy ) from 0 to ( x^2+y^2. )
(displaystyle = int_{-a}^a int_{-sqrt{a^2-x^2}}^{sqrt{a^2-x^2}} frac{(x^2+y^2 )^3}{3} dxdy)
( = frac{1}{3} int_0^π int_{-a}^a r^7 drdθ )
( = π frac{a^8}{12}. )
3. Find the volume of the cylinder bounded by x2+y2 = 4, y+z = 4 and z=0.
a) (16π-frac{32}{3} )
b) (32π-frac{32}{3} )
c) (16-32 frac{π}{3} )
d) (32-32 frac{π}{3} )
Answer: a
Explanation: (z: (4-y))
(x: -sqrt{4-y^2} ) to ( sqrt{4-y^2} )
y: -2 to 2
Volume = (displaystyleint_{-2}^2 int_{-sqrt{4-y^2}}^{sqrt{4-y^2}} (4-y)dxdy )
(= 2int_{-2}^2 (4y-y^2 )dxdy ) from 0 to ( sqrt{4-y^2} )
(= 4big{4(0 + π) – 4 + frac{4}{3}big} )
(= 16π-frac{32}{3}. )
4. Find the volume of sphere by triple integration.
a) (8a^3 frac{π}{3} )
b) (4a^3 frac{π}{3} )
c) (2a^3 frac{π}{3} )
d) (a^3 frac{π}{3} )
Answer: b
Explanation: The sphere is given by x2+y2+z2=a2.
θ : 0 to 2 π
φ : 0 to π
r : 0 to a
Volume = ( int_0^a int_0^π int_0^{2π} r^2 sinθdθdrdϕ )
(= 4(int_0^a r^2 dr)(int_0^π sinθdθ)(int_0^{π/2}dϕ) )
(= 4a^3 frac{π}{3}.)
5. Using polar coordinates, find the volume of the cylinder with radius a and height h.
a) (πa^2h )
b) (frac{πa^2h}{3} )
c) (2 frac{πa^2h}{3} )
d) ( 4 frac{πa^2h}{3} )
Answer: a
Explanation: r: 0 to a
θ : 0 to 2 π
z: 0 to h
Therefore, volume = (int_0^a int_0^{2π}int_0^h rdθdrdz )
(= (int_0^a rdr)(int_0^{2π}dθ)(int_0^h dz) )
(= frac{a^2}{2} * 2π * h )
(= πa^2h. )
6. In multiple integrals, if the limits depends on variable, then the order of integration can be anything.
a) True
b) False
Answer: b
Explanation: In multiple integration, if the limits depend on variable then the order of integration can’t be anything. First the dependent integration should be done and then the independent integration.
7. To find volume _________________ can be used.
a) single integration
b) double integration
c) triple integration
d) double & triple integration
Answer: d
Explanation: To find volume, triple integration should be used and proper limits should be given for each variable.
But volume integration can also be done using double integration by using 1D equation of the 3D object as the function.
8. Evaluate (int_{-1}^1 int_0^z int_{x-z}^{x+z}(x+y+z)dxdydz.)
a) 0
b) 1
c) 2
d) 3
Answer: a
Explanation: (int_{-1}^1 int_0^z int_{x-z}^{x+z}(x+y+z)dxdydz=
int_{-1}^1 int_0^z (xy+frac{y^2}{2}+zy)dxdz ) from x-z to x+z
(= int_{-1}^1int_0^z(3x^2+z^2+2xz)dxdz= int_{-1}^1(x^3+z^2 x+x^2 z)dz ) from 0 to z
(= int_{-1}^1(z^3+z^3+z^3)dz= frac{z^4}{4} ) from -1 to 1
= 0.
9. Evaluate (int_0^1int_0^{sqrt{1-x^2}}int_0^{sqrt{1-x^2-y^2}}xyzdxdydz.)
a) (frac{1}{6} )
b) (frac{1}{12} )
c) (frac{1}{24} )
d) (frac{1}{48} )
Answer: c
Explanation: (int_0^1 int_0^{sqrt{1-x^2}}int_0^{sqrt{1-x^2-y^2}}xyzdxdydz=
int_0^1 int_0^{sqrt{1-x^2}} frac{xyz^2}{2} dxdy ) from 0 to ( sqrt{1-x^2-y^2} )
(= frac{1}{2} int_0^1 frac{xy^2}{2} – frac{x}{2} – frac{xy^4}{4} dx ) from 0 to (sqrt{1-x^2} )
(= frac{1}{2} int_0^1 frac{(x-x^3)}{2}-frac{(x^3-x^5)}{2}-frac{(x+x^5-2x^3)}{4} dx)
(= frac{1}{4}big{frac{1}{6} – frac{1}{2} + frac{1}{2}big} )
(= frac{1}{24}. )
10. What is the volume of a cube with side a?
a) (frac{a^3}{8} )
b) (a^2)
c) (a^3)
d) (frac{a2}{4} )
Answer: c
Explanation: (int_0^a int_0^a int_0^a dxdydz = (int_0^a dx)(int_0^a dy)(int_0^a dz)= a^3.)
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