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Volumes of Revolution

12 practice questions with full worked solutions. Free, no account needed.

Problems & worked solutions

Problem #1554 International

Problem 1 Shell Method

The region bounded by the graph of f(x)=x2, the Ox axis, and the line x=2 (for x[0,2]) is rotated about the Oy axis. Using the shell method (V=2π02xf(x)dx), the volume of the resulting solid is:

Show answer & worked solution
  1. A. 4π
  2. B. 16π
  3. C. 8π✓ correct
  4. D. 32π5

We apply the shell method formula directly (rotation about Oy): V=2π02xx2dx=2π02x3dx=2π[x44]02=2π4=8π.

Check: [x44]02=164=4, so V=2π4=8π ✓ (the option 4π appears if you drop the factor of 2 from the formula).

Problem #0573 International

Problem 2 Volumes of Revolution

The volume generated by rotating f(x)=x on [0,3] about the x-axis is:

Show answer & worked solution
  1. A. 3π
  2. B. 6π
  3. C. 9π✓ correct
  4. D. 27π

V=π03x2dx=π273=9π. (Equivalently, Vcone=13πr2h=13π93=9π.)

Problem #0572 International

Problem 3 Volumes of Revolution

The volume generated by rotating f(x)=2 on [0,5] about the x-axis is:

Show answer & worked solution
  1. A. 5π
  2. B. 10π
  3. C. 20π✓ correct
  4. D. 40π

Cylinder of radius 2 and height 5: V=π45=20π.

Problem #0571 International

Problem 4 Volumes of Revolution

The volume of the solid generated by rotating f(x)0 on [a,b] about the x-axis is:

Show answer & worked solution
  1. A. πabf(x)dx
  2. B. abf(x)2dx
  3. C. πab[f(x)]2dx✓ correct
  4. D. 2πabf(x)dx

V=πab[f(x)]2dx — the disk-method formula.

Problem #0578 International

Problem 5 Volumes of Revolution

The volume generated by rotating f(x)=cosx on [0,π/2] about the x-axis is closest to:

Show answer & worked solution
  1. A. π2
  2. B. π24✓ correct
  3. C. π22
  4. D. π

V=π0π/2cos2xdx=π0π/21+cos2x2dx=π2 ⁣[x+sin2x2]0π/2=π2π2=π24.

Problem #0577 International

Problem 6 Volumes of Revolution

The volume generated by rotating f(x)=ex on [0,1] about the x-axis is:

Show answer & worked solution
  1. A. π(e1)
  2. B. πe
  3. C. π2(e21)✓ correct
  4. D. πe2

V=π01(ex)2dx=π01e2xdx=πe2x201=π2(e21).

Problem #0574 International

Problem 7 Volumes of Revolution

The volume generated by rotating f(x)=x on [0,4] about the x-axis is:

Show answer & worked solution
  1. A. 4π
  2. B. 8π✓ correct
  3. C. 12π
  4. D. 16π

V=π04(x)2dx=π04xdx=π162=8π.

Problem #0576 International

Problem 8 Volumes of Revolution

The volume generated by rotating f(x)=x2 on [0,1] about the x-axis is:

Show answer & worked solution
  1. A. π
  2. B. π3
  3. C. π5✓ correct
  4. D. π7

V=π01x4dx=π15=π5.

Problem #0575 International

Problem 9 Volumes of Revolution

Rotating f(x)=R2x2 on [R,R] about the x-axis generates a:

Show answer & worked solution
  1. A. cone
  2. B. cylinder
  3. C. sphere of radius R✓ correct
  4. D. ellipsoid

The graph is a semicircle; rotating about the x-axis sweeps out a sphere of radius R. (Volume =43πR3.)

Problem #0579 International

Problem 10 Volumes of Revolution

The volume of the solid generated by rotating the region between y=x and y=x2 on [0,1] about the x-axis equals:

Show answer & worked solution
  1. A. π/30
  2. B. π/15
  3. C. 2π/15✓ correct
  4. D. π/3

V=π01(x2x4)dx=π ⁣[x33x55]01=π ⁣(1315)=2π15.

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