Math Problem Statement

Use the shell method to find the volume of the solid generated by revolving the region bounded by the graphs of the equation about the given line.

y = 6x − x2,      y = 0,

about the line x = 8

Solution

We are tasked with finding the volume of the solid generated by revolving the region bounded by the curves y=6xx2y = 6x - x^2 and y=0y = 0 about the line x=8x = 8. To achieve this, we will use the shell method.

Step-by-Step Solution:

  1. Identify the region: The curves that bound the region are:

    • y=6xx2y = 6x - x^2, a parabola opening downwards.
    • y=0y = 0, the x-axis.

    We need to find the intersection points of these curves, i.e., solve: 6xx2=06x - x^2 = 0 Factoring: x(6x)=0x(6 - x) = 0 So, x=0x = 0 and x=6x = 6.

  2. Set up the shell method formula: The volume using the shell method is given by: V=2πab(radius)×(height)dxV = 2\pi \int_{a}^{b} ( \text{radius} ) \times ( \text{height} ) \, dx where:

    • The radius is the horizontal distance from the curve to the line of rotation x=8x = 8. This distance is 8x8 - x.
    • The height of the shell is given by the value of yy, which is 6xx26x - x^2.

    Therefore, the volume integral becomes: V=2π06(8x)(6xx2)dxV = 2\pi \int_{0}^{6} (8 - x)(6x - x^2) \, dx

  3. Expand the integrand: Let's expand the expression inside the integral: (8x)(6xx2)=8(6xx2)x(6xx2)(8 - x)(6x - x^2) = 8(6x - x^2) - x(6x - x^2) Simplifying both terms: =48x8x26x2+x3= 48x - 8x^2 - 6x^2 + x^3 =48x14x2+x3= 48x - 14x^2 + x^3

  4. Evaluate the integral: Now, we integrate term by term: V=2π06(48x14x2+x3)dxV = 2\pi \int_{0}^{6} (48x - 14x^2 + x^3) \, dx

    • The integral of 48x48x is 24x224x^2,
    • The integral of 14x2-14x^2 is 143x3-\frac{14}{3}x^3,
    • The integral of x3x^3 is x44\frac{x^4}{4}.

    So, the volume is: V=2π[24x2143x3+x44]06V = 2\pi \left[ 24x^2 - \frac{14}{3}x^3 + \frac{x^4}{4} \right]_{0}^{6}

  5. Substitute the limits: First, plug in x=6x = 6:

    = 24(36) - \frac{14}{3}(216) + \frac{1296}{4} = 864 - 1008 + 324 = 180$$ When $$x = 0$$, all terms vanish. Therefore, the volume is: $$V = 2\pi \times 180 = 360\pi$$

Final Answer:

The volume of the solid is 360π\boxed{360\pi} cubic units.

Would you like further clarification on any part of the solution? Here are some related questions you can explore:

  1. How does the shell method compare to the disk/washer method in this type of problem?
  2. What if the line of rotation were x=8x = -8? How would the setup change?
  3. How do you find the volume when the region is rotated about the y-axis using the shell method?
  4. What if the parabola had a different equation, say y=5xx2y = 5x - x^2?
  5. How does the choice of the axis of rotation affect the resulting solid's shape?

Tip: When using the shell method, the radius is always the distance from the axis of rotation to the representative shell. Keep this in mind when setting up the problem!

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Math Problem Analysis

Mathematical Concepts

Calculus
Volume of Solids of Revolution
Shell Method
Quadratic Functions

Formulas

V = 2π ∫(radius) × (height) dx
Quadratic equation: y = 6x - x^2

Theorems

Shell Method for Volume Calculation

Suitable Grade Level

Grade 12 or College Calculus