Math Problem Statement

The region bounded by the given curves is rotated about the specified axis. Find the volume of the resulting solid by any method. y = −x2 + 8x − 15, y = 0; about the x-axis

Solution

To find the volume of the solid formed by rotating the region bounded by the curves y=x2+8x15y = -x^2 + 8x - 15 and y=0y = 0 about the x-axis, we will use the disk method. The disk method involves integrating the area of circular disks whose radius is given by the function y=f(x)y = f(x) over the interval of interest.

Step 1: Determine the points of intersection

First, let's find the points where the curve intersects the x-axis, i.e., where y=0y = 0.

Set the equation equal to zero:

x2+8x15=0-x^2 + 8x - 15 = 0

Solve this quadratic equation. We can use the quadratic formula:

x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} where a=1a = -1, b=8b = 8, and c=15c = -15. Substituting these values:

x=8±824(1)(15)2(1)x = \frac{-8 \pm \sqrt{8^2 - 4(-1)(-15)}}{2(-1)} x=8±64602x = \frac{-8 \pm \sqrt{64 - 60}}{-2} x=8±42x = \frac{-8 \pm \sqrt{4}}{-2} x=8±22x = \frac{-8 \pm 2}{-2}

So, the two solutions are:

x=8+22=3andx=822=5x = \frac{-8 + 2}{-2} = 3 \quad \text{and} \quad x = \frac{-8 - 2}{-2} = 5

Thus, the points of intersection are at x=3x = 3 and x=5x = 5.

Step 2: Set up the volume integral

The formula for the volume using the disk method is:

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

In this case, f(x)=x2+8x15f(x) = -x^2 + 8x - 15, and the limits of integration are from x=3x = 3 to x=5x = 5. So, the volume integral becomes:

V=π35(x2+8x15)2dxV = \pi \int_{3}^{5} \left( -x^2 + 8x - 15 \right)^2 \, dx

Step 3: Simplify and compute the integral

First, expand (x2+8x15)2\left( -x^2 + 8x - 15 \right)^2:

(x2+8x15)2=(x28x+15)2\left( -x^2 + 8x - 15 \right)^2 = (x^2 - 8x + 15)^2 =(x2)22(x2)(8x)+2(x2)(15)+(8x)22(8x)(15)+152= (x^2)^2 - 2(x^2)(8x) + 2(x^2)(15) + (8x)^2 - 2(8x)(15) + 15^2 =x416x3+106x2240x+225= x^4 - 16x^3 + 106x^2 - 240x + 225

Now, integrate the expanded polynomial:

V=π35(x416x3+106x2240x+225)dxV = \pi \int_{3}^{5} (x^4 - 16x^3 + 106x^2 - 240x + 225) \, dx

Integrating term by term:

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:

V=π[x554x4+106x33120x2+225x]35V = \pi \left[ \frac{x^5}{5} - 4x^4 + \frac{106x^3}{3} - 120x^2 + 225x \right]_{3}^{5}

Step 4: Evaluate the integral at the limits

Now evaluate the integral at x=5x = 5 and x=3x = 3:

At x=5x = 5:

V=π[5554(54)+106(53)3120(52)+225(5)]V = \pi \left[ \frac{5^5}{5} - 4(5^4) + \frac{106(5^3)}{3} - 120(5^2) + 225(5) \right]

At x=3x = 3:

V=π[3554(34)+106(33)3120(32)+225(3)]V = \pi \left[ \frac{3^5}{5} - 4(3^4) + \frac{106(3^3)}{3} - 120(3^2) + 225(3) \right]

Subtract the values to find the volume.

Final Answer

The result of the above subtraction will give you the volume of the solid formed by rotating the given region about the x-axis.

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Math Problem Analysis

Mathematical Concepts

Calculus
Volume of Solids of Revolution
Quadratic Equations

Formulas

Disk Method: V = π ∫[a, b] [f(x)]² dx
Quadratic Equation: ax² + bx + c = 0

Theorems

Fundamental Theorem of Calculus
Quadratic Formula

Suitable Grade Level

College Calculus (AP Calculus or equivalent)