Math Problem Statement

  1. Find the volume of the solid obtained by rotating the region bounded by y = 3 \cos(x^2),y=0 ,x= 0 , x = \sqrt{\displaystyle\frac{\pi}{3}} and about the -axis.

Solution

To find the volume of the solid obtained by rotating the region bounded by the curves y=3cos(x2)y = 3 \cos(x^2), y=0y = 0, x=0x = 0, and x=π3x = \sqrt{\frac{\pi}{3}} about the xx-axis, we can use the method of disks.

Step 1: Set up the volume integral

When rotating a region about the xx-axis, the volume VV is given by:

V=πab[f(x)]2dxV = \pi \int_{a}^{b} \left[ f(x) \right]^2 \, dx

Here:

  • f(x)=3cos(x2)f(x) = 3 \cos(x^2)
  • The bounds are x=0x = 0 to x=π3x = \sqrt{\frac{\pi}{3}}.

So the volume integral becomes:

V=π0π3[3cos(x2)]2dxV = \pi \int_{0}^{\sqrt{\frac{\pi}{3}}} \left[ 3 \cos(x^2) \right]^2 \, dx

Step 2: Simplify the integrand

Simplify the integrand:

V=π0π39cos2(x2)dxV = \pi \int_{0}^{\sqrt{\frac{\pi}{3}}} 9 \cos^2(x^2) \, dx

Factor out the constant:

V=9π0π3cos2(x2)dxV = 9\pi \int_{0}^{\sqrt{\frac{\pi}{3}}} \cos^2(x^2) \, dx

Step 3: Use an appropriate substitution or numerical methods

The integral cos2(x2)dx\int \cos^2(x^2) \, dx does not have an elementary antiderivative. Therefore, this integral is typically evaluated using numerical methods.

However, we can express the integral using the half-angle identity for cosine:

cos2(θ)=1+cos(2θ)2\cos^2(\theta) = \frac{1 + \cos(2\theta)}{2}

For our integral:

cos2(x2)=1+cos(2x2)2\cos^2(x^2) = \frac{1 + \cos(2x^2)}{2}

Thus, the integral becomes:

V=9π0π31+cos(2x2)2dxV = 9\pi \int_{0}^{\sqrt{\frac{\pi}{3}}} \frac{1 + \cos(2x^2)}{2} \, dx

Simplifying:

V=9π20π3[1+cos(2x2)]dxV = \frac{9\pi}{2} \int_{0}^{\sqrt{\frac{\pi}{3}}} \left[ 1 + \cos(2x^2) \right] \, dx

This integral can be split into two parts:

V=9π2[0π31dx+0π3cos(2x2)dx]V = \frac{9\pi}{2} \left[ \int_{0}^{\sqrt{\frac{\pi}{3}}} 1 \, dx + \int_{0}^{\sqrt{\frac{\pi}{3}}} \cos(2x^2) \, dx \right]

The first integral is straightforward:

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

The second integral 0π3cos(2x2)dx\int_{0}^{\sqrt{\frac{\pi}{3}}} \cos(2x^2) \, dx does not have a simple closed form and is usually evaluated numerically.

Let's denote this integral by II:

I=0π3cos(2x2)dxI = \int_{0}^{\sqrt{\frac{\pi}{3}}} \cos(2x^2) \, dx

Thus, the volume VV becomes:

V=9π2[π3+I]V = \frac{9\pi}{2} \left[ \sqrt{\frac{\pi}{3}} + I \right]

Step 4: Compute the numerical value

The integral II can be computed numerically, but we can provide a simplified expression here:

V=9π2[π3+I]V = \frac{9\pi}{2} \left[ \sqrt{\frac{\pi}{3}} + I \right]

Since II is a definite integral, it is a fixed number that can be approximated using numerical integration methods such as Simpson's rule, trapezoidal rule, or using a calculator or software.

Final Answer:

The volume of the solid obtained by rotating the region bounded by y=3cos(x2)y = 3 \cos(x^2), y=0y = 0, x=0x = 0, and x=π3x = \sqrt{\frac{\pi}{3}} about the xx-axis is given by:

V=9π2[π3+I]V = \frac{9\pi}{2} \left[ \sqrt{\frac{\pi}{3}} + I \right]

where II is the integral 0π3cos(2x2)dx\int_{0}^{\sqrt{\frac{\pi}{3}}} \cos(2x^2) \, dx, which needs to be evaluated numerically for an exact value.

If you need the exact numerical value, I can help compute it. Let me know!

Here are 5 related questions:

  1. How do you approach integrals that do not have elementary antiderivatives?
  2. What is the half-angle identity, and how is it used in integration?
  3. How can you evaluate integrals numerically?
  4. What are some alternative methods for finding volumes of revolution?
  5. How does the choice of axis of rotation affect the integral setup?

Tip: When dealing with complex integrals that do not have elementary solutions, consider using numerical methods or computer algebra systems to approximate the result.

Ask a new question for Free

By Image

Drop file here or Click Here to upload

Math Problem Analysis

Mathematical Concepts

Volume of Revolution
Integration
Trigonometric Substitution

Formulas

Volume integral using disks/washers

Theorems

-

Suitable Grade Level

University Level