Math Problem Statement

Find the volume of the solid generated by rotating the region y = sin(3x), y = 0, x = 0, and x = π/3 about the x-axis.

Solution

To find the volume of the solid generated by rotating the region bounded by y=sin(3x)y = \sin(3x), y=0y = 0, x=0x = 0, and x=π3x = \frac{\pi}{3} about the xx-axis, we use the method of disks. The formula for the volume VV when the region between the curve y=f(x)y = f(x) and the xx-axis from x=ax = a to x=bx = b is rotated about the xx-axis is given by:

V=πab[f(x)]2dxV = \pi \int_a^b [f(x)]^2 \, dx

For the given function f(x)=sin(3x)f(x) = \sin(3x), the limits of integration are a=0a = 0 and b=π3b = \frac{\pi}{3}. Substituting these into the volume formula, we have:

V=π0π3[sin(3x)]2dxV = \pi \int_0^{\frac{\pi}{3}} [\sin(3x)]^2 \, dx

To simplify the integral, we can use the trigonometric identity sin2(θ)=1cos(2θ)2\sin^2(\theta) = \frac{1 - \cos(2\theta)}{2}:

[sin(3x)]2=1cos(6x)2[\sin(3x)]^2 = \frac{1 - \cos(6x)}{2}

The volume integral becomes:

V=π0π31cos(6x)2dxV = \pi \int_0^{\frac{\pi}{3}} \frac{1 - \cos(6x)}{2} \, dx

This simplifies to:

V=π20π3(1cos(6x))dxV = \frac{\pi}{2} \int_0^{\frac{\pi}{3}} (1 - \cos(6x)) \, dx

We can now compute this integral:

0π31dx=π3\int_0^{\frac{\pi}{3}} 1 \, dx = \frac{\pi}{3}

0π3cos(6x)dx=16sin(6x)0π3=16(sin(2π)sin(0))=0\int_0^{\frac{\pi}{3}} \cos(6x) \, dx = \frac{1}{6} \sin(6x) \Big|_0^{\frac{\pi}{3}} = \frac{1}{6}(\sin(2\pi) - \sin(0)) = 0

Thus, the volume is:

V=π2(π30)=π26V = \frac{\pi}{2} \left(\frac{\pi}{3} - 0\right) = \frac{\pi^2}{6}

The volume of the solid generated is π26\frac{\pi^2}{6} cubic units.

Would you like more details on this calculation or help with another problem?

Here are some related questions to consider:

  1. What would be the volume if the same region were rotated about the y-axis?
  2. How does changing the limits of integration affect the volume of the solid?
  3. What would the volume be if y=sin(4x)y = \sin(4x) instead?
  4. Can you calculate the volume using cylindrical shells instead of disks for this problem?
  5. How does the integral change if the function involves cosine instead of sine?

Tip: When evaluating trigonometric integrals, recall identities and properties to simplify integrals wherever possible.

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Math Problem Analysis

Mathematical Concepts

Calculus
Volume of Solids
Definite Integral
Trigonometric Functions

Formulas

Volume of a solid generated by rotation around the x-axis: V = π ∫ [f(x)]^2 dx
Trigonometric identity: sin^2(θ) = (1 - cos(2θ))/2

Theorems

Disk method for finding volume

Suitable Grade Level

College-level Calculus