Math Problem Statement

You want to find the volume of the solid obtained by rotating about the x-axis the region under the curve y=x1/5𝑦=𝑥1/5 from 0 to 2.

You first slice through the rotated solid at a generic point x𝑥 and get a circular cross-section. What is the area of the circular cross-section?

A(x)=𝐴(𝑥)=πx25​Correct  

This makes the volume of the approximating disk with thickness ΔxΔ𝑥 equal to which expression?

Volume of disk = πx25​Correct ΔxΔ𝑥  

Now let ΔxΔ𝑥 approach 0, and sum the volumes of the infinitely many disks that approximate the solid of revolution. What total volume do you get?

V=∫20A(x)dx=

Solution

To find the total volume VV of the solid obtained by rotating the region under the curve y=x1/5y = x^{1/5} around the x-axis from x=0x = 0 to x=2x = 2, we follow these steps:

Step 1: Area of the circular cross-section

The area of a circular cross-section at a generic point xx is given by: A(x)=π(y(x))2=π(x1/5)2=πx2/5.A(x) = \pi \left( y(x) \right)^2 = \pi \left( x^{1/5} \right)^2 = \pi x^{2/5}.

Step 2: Volume of an approximating disk

The volume of an approximating disk at a point xx with a small thickness Δx\Delta x is: Volume of disk=A(x)Δx=πx2/5Δx.\text{Volume of disk} = A(x) \Delta x = \pi x^{2/5} \Delta x.

Step 3: Total volume of the solid

To find the total volume, sum up the volumes of the disks as Δx\Delta x approaches 0. This is done by integrating the area function A(x)A(x) from x=0x = 0 to x=2x = 2: V=02A(x)dx=02πx2/5dx.V = \int_0^2 A(x) \, dx = \int_0^2 \pi x^{2/5} \, dx. We can factor out the constant π\pi: V=π02x2/5dx.V = \pi \int_0^2 x^{2/5} \, dx.

Step 4: Compute the integral

To compute 02x2/5dx\int_0^2 x^{2/5} \, dx, use the power rule of integration: xndx=xn+1n+1,where n=25.\int x^n \, dx = \frac{x^{n+1}}{n+1}, \quad \text{where } n = \frac{2}{5}. So, 02x2/5dx=[x7/57/5]02=57(27/507/5).\int_0^2 x^{2/5} \, dx = \left[ \frac{x^{7/5}}{7/5} \right]_0^2 = \frac{5}{7} \left( 2^{7/5} - 0^{7/5} \right). Since 07/5=00^{7/5} = 0, this simplifies to: 02x2/5dx=5727/5.\int_0^2 x^{2/5} \, dx = \frac{5}{7} \cdot 2^{7/5}.

Step 5: Final volume expression

Now, multiplying by π\pi: V=π5727/5.V = \pi \cdot \frac{5}{7} \cdot 2^{7/5}.

Thus, the total volume of the solid is: V=5π727/5.V = \frac{5\pi}{7} \cdot 2^{7/5}.

Would you like a numerical evaluation of this result or more details on any step?


Here are 5 related questions to expand on this concept:

  1. How would the volume change if the curve was rotated about the y-axis instead?
  2. What is the volume if the curve is y=xy = \sqrt{x} from x=0x = 0 to x=2x = 2?
  3. How does the integral setup change if we rotate around a horizontal line y=cy = c?
  4. How can we find the surface area of the same solid instead of the volume?
  5. What happens if we rotate the region under the curve about the line x=3x = 3?

Tip: For solids of revolution, always check if the cross-section is perpendicular to the axis of rotation to correctly apply the disk or washer method.

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Math Problem Analysis

Mathematical Concepts

Calculus
Solids of Revolution
Disk Method
Integration

Formulas

Area of cross-section: A(x) = π(x^(2/5))
Volume of disk: π(x^(2/5))Δx
Total volume: V = π ∫(0 to 2) x^(2/5) dx

Theorems

Disk Method
Power Rule of Integration

Suitable Grade Level

College-level Calculus