Math Problem Statement

Find the volume V of the solid obtained by rotating the region bounded by the given curves about the specified line.

y =

x − 1

,  y = 0,  x = 8;    about the x-axis

Solution

To find the volume VV of the solid obtained by rotating the region bounded by the curves about the xx-axis, we can apply the Disk Method.

Step-by-Step Approach:

  1. Identify the curves and region:

    • The region is bounded by:
      • y=x1y = x - 1
      • y=0y = 0 (the x-axis)
      • x=8x = 8
  2. Set up the Disk Method formula:

    The volume of a solid obtained by rotating a region about the xx-axis is given by: V=πab[f(x)]2dxV = \pi \int_{a}^{b} [f(x)]^2 \, dx where f(x)f(x) is the function describing the curve being rotated, and [a,b][a, b] are the limits of integration along the xx-axis.

  3. Set up the integral:

    The curve y=x1y = x - 1 will be rotated about the xx-axis. This gives the radius of each disk as f(x)=x1f(x) = x - 1. The limits of integration are from x=1x = 1 (where y=0y = 0) to x=8x = 8 (as given in the problem).

    So, the integral becomes: V=π18(x1)2dxV = \pi \int_{1}^{8} (x - 1)^2 \, dx

  4. Expand the integrand:

    First, expand (x1)2(x - 1)^2: (x1)2=x22x+1(x - 1)^2 = x^2 - 2x + 1 Now, the integral becomes: V=π18(x22x+1)dxV = \pi \int_{1}^{8} (x^2 - 2x + 1) \, dx

  5. Integrate:

    Now, integrate each term: V=π[x33x2+x]18V = \pi \left[ \frac{x^3}{3} - x^2 + x \right]_{1}^{8}

  6. Evaluate the definite integral:

    First, evaluate at x=8x = 8: (83382+8)=(512364+8)=512356=5121683=3443\left( \frac{8^3}{3} - 8^2 + 8 \right) = \left( \frac{512}{3} - 64 + 8 \right) = \frac{512}{3} - 56 = \frac{512 - 168}{3} = \frac{344}{3}

    Next, evaluate at x=1x = 1: (13312+1)=(131+1)=13\left( \frac{1^3}{3} - 1^2 + 1 \right) = \left( \frac{1}{3} - 1 + 1 \right) = \frac{1}{3}

    Now, subtract the two values: V=π(344313)=π(3433)V = \pi \left( \frac{344}{3} - \frac{1}{3} \right) = \pi \left( \frac{343}{3} \right)

  7. Final answer:

    The volume of the solid is: V=343π3V = \frac{343\pi}{3}

Do you need further details or clarifications?


Here are 5 related questions to expand on the topic:

  1. How does the Disk Method differ from the Shell Method?
  2. What would the volume be if the region were rotated about the line y=1y = 1?
  3. Can you solve the same problem using cylindrical shells instead of disks?
  4. How would the volume change if the curve were rotated about the yy-axis?
  5. How do you find the volume of a solid with a more complex boundary, such as y=x2y = x^2?

Tip: Always check the limits of integration carefully when setting up an integral for volumes of rotation.

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Math Problem Analysis

Mathematical Concepts

Integral Calculus
Volumes of Solids of Revolution
Disk Method

Formulas

Volume of solid of revolution: V = π ∫[a to b] [f(x)]^2 dx
f(x) = x - 1

Theorems

Disk Method

Suitable Grade Level

Grades 11-12