Math Problem Statement

Find the volume of the solid obtained by rotating the region bounded by the given curves about the x-axis.

y = 8 − 8x2, y = 0

Solution

To find the volume of the solid obtained by rotating the region bounded by the curves y=88x2y = 8 - 8x^2 and y=0y = 0 about the xx-axis, we use the disk method.

The volume VV of the solid formed by revolving a curve y=f(x)y = f(x) around the xx-axis from x=ax = a to x=bx = b is given by the formula:

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

Here, f(x)=88x2f(x) = 8 - 8x^2 and y=0y = 0 is the boundary, so the region we're rotating lies between these two. Now let's go through the steps:

Step 1: Find the bounds aa and bb

We need to find the xx-values where the curve intersects the xx-axis. This happens when y=0y = 0, so:

88x2=08 - 8x^2 = 0 8x2=8    x2=1    x=±18x^2 = 8 \implies x^2 = 1 \implies x = \pm 1

Thus, the limits of integration are x=1x = -1 and x=1x = 1.

Step 2: Set up the integral

Using the formula for the disk method, we square f(x)f(x) and set up the integral:

V=π11(88x2)2dxV = \pi \int_{-1}^{1} (8 - 8x^2)^2 \, dx

Step 3: Solve the integral

First, expand the square:

(88x2)2=64128x2+64x4(8 - 8x^2)^2 = 64 - 128x^2 + 64x^4

Now, integrate term by term:

V=π11(64128x2+64x4)dxV = \pi \int_{-1}^{1} \left( 64 - 128x^2 + 64x^4 \right) dx

Step 4: Compute the integrals

Since the function is even (symmetric around the yy-axis), we can double the integral from 0 to 1 to simplify the calculation:

V=2π01(64128x2+64x4)dxV = 2\pi \int_{0}^{1} \left( 64 - 128x^2 + 64x^4 \right) dx

Now, calculate each integral:

64dx=64x\int 64 \, dx = 64x 128x2dx=128x33\int -128x^2 \, dx = -\frac{128x^3}{3} 64x4dx=64x55\int 64x^4 \, dx = \frac{64x^5}{5}

Evaluating these from 0 to 1:

64(1)128(13)3+64(15)5=641283+64564(1) - \frac{128(1^3)}{3} + \frac{64(1^5)}{5} = 64 - \frac{128}{3} + \frac{64}{5}

Now, simplify this expression:

641283+645=9601564015+19215=5121564 - \frac{128}{3} + \frac{64}{5} = \frac{960}{15} - \frac{640}{15} + \frac{192}{15} = \frac{512}{15}

Finally, multiply by 2π2\pi:

V=2π×51215=1024π15V = 2\pi \times \frac{512}{15} = \frac{1024\pi}{15}

Final Answer:

The volume of the solid is:

V=1024π15cubic unitsV = \frac{1024\pi}{15} \, \text{cubic units}

Would you like further details or clarifications?


Here are some related questions you might explore:

  1. How does the disk method differ from the washer method?
  2. What happens if the region is rotated about a different axis, like the y-axis?
  3. Can the volume be computed using cylindrical shells instead?
  4. What is the effect of changing the equation to y=84x2y = 8 - 4x^2?
  5. How does the method work when revolving around a non-axis line?

Tip: Symmetry often simplifies integrals; use it when functions are even!

Ask a new question for Free

By Image

Drop file here or Click Here to upload

Math Problem Analysis

Mathematical Concepts

Calculus
Volume of Solids of Revolution
Disk Method

Formulas

V = π ∫[a,b] [f(x)]^2 dx
f(x) = 8 - 8x^2

Theorems

Disk Method

Suitable Grade Level

Grades 11-12