Math Problem Statement

The region bounded by the given curves is rotated about the specified axis. Find the volume V of the resulting solid by any method. y = −x^2 + 10x − 16, y = 0; about the x-axis

Solution

We are tasked with finding the volume of the solid formed by rotating the region bounded by the curve y=x2+10x16y = -x^2 + 10x - 16 and the line y=0y = 0 (the x-axis) about the x-axis.

Step 1: Find the bounds

We first need to determine where the curve intersects the x-axis. This is done by solving for xx when y=0y = 0:

0=x2+10x160 = -x^2 + 10x - 16

This is a quadratic equation. We can solve it using the quadratic formula:

x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

For the equation x2+10x16=0-x^2 + 10x - 16 = 0, we have a=1a = -1, b=10b = 10, and c=16c = -16. Plugging these values into the quadratic formula:

x=10±1024(1)(16)2(1)x = \frac{-10 \pm \sqrt{10^2 - 4(-1)(-16)}}{2(-1)} x=10±100642x = \frac{-10 \pm \sqrt{100 - 64}}{-2} x=10±362x = \frac{-10 \pm \sqrt{36}}{-2} x=10±62x = \frac{-10 \pm 6}{-2}

This gives two solutions:

x1=10+62=2,x2=1062=8x_1 = \frac{-10 + 6}{-2} = 2, \quad x_2 = \frac{-10 - 6}{-2} = 8

So, the curve intersects the x-axis at x=2x = 2 and x=8x = 8.

Step 2: Set up the volume integral (Disk method)

The volume of the solid formed by rotating the region about the x-axis can be found using the disk method. The formula for the volume is:

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

In our case, f(x)=x2+10x16f(x) = -x^2 + 10x - 16, and the limits of integration are from x=2x = 2 to x=8x = 8. So the volume is:

V=π28[x2+10x16]2dxV = \pi \int_{2}^{8} \left[ -x^2 + 10x - 16 \right]^2 \, dx

Step 3: Simplify the integrand

We now expand (x2+10x16)2(-x^2 + 10x - 16)^2:

(x2+10x16)2=(x420x3+132x2320x+256)\left(-x^2 + 10x - 16\right)^2 = (x^4 - 20x^3 + 132x^2 - 320x + 256)

Step 4: Perform the integration

We integrate each term:

28(x420x3+132x2320x+256)dx\int_{2}^{8} \left( x^4 - 20x^3 + 132x^2 - 320x + 256 \right) \, dx

Each term is integrated as follows:

x4dx=x55,x3dx=x44,x2dx=x33,xdx=x22,1dx=x\int x^4 \, dx = \frac{x^5}{5}, \quad \int x^3 \, dx = \frac{x^4}{4}, \quad \int x^2 \, dx = \frac{x^3}{3}, \quad \int x \, dx = \frac{x^2}{2}, \quad \int 1 \, dx = x

Thus, the integral becomes:

V=π[x555x4+44x3160x2+256x]28V = \pi \left[ \frac{x^5}{5} - 5x^4 + 44x^3 - 160x^2 + 256x \right]_{2}^{8}

Step 5: Compute the definite integral

We now evaluate this expression at x=8x = 8 and x=2x = 2 and subtract the results:

For x=8x = 8:

[ \frac{8^5}{5} - 5(

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Math Problem Analysis

Mathematical Concepts

Algebra
Quadratic Equations
Volume of Solids of Revolution
Disk Method

Formulas

Quadratic equation: ax^2 + bx + c = 0
Disk method for volume: V = π ∫[a to b] [f(x)]^2 dx
Quadratic formula: x = (-b ± √(b^2 - 4ac)) / 2a

Theorems

Quadratic formula
Disk method

Suitable Grade Level

Grades 10-12