Math Problem Statement

Let R be the region bounded by the curves y = -x^2+6x-5 and y = 0. Let S be the solid formed by revolving R about the x axis. Select and use the most appropriate method to find the volume of S. If necessary, round to 4 decimal places.

Solution

To find the volume of the solid SS, we use the disk method because the region RR is revolved around the x-axis. The volume of a solid of revolution formed by revolving a region around the x-axis is given by:

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

Step 1: Identify the region boundaries

The curve is y=x2+6x5y = -x^2 + 6x - 5, and we are revolving the region bounded by this curve and y=0y = 0 (the x-axis).

To find the limits of integration aa and bb, we need to determine where the curve intersects the x-axis (i.e., where y=0y = 0):

x2+6x5=0-x^2 + 6x - 5 = 0

This is a quadratic equation, which we can solve using the quadratic formula:

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

For x2+6x5=0-x^2 + 6x - 5 = 0, we have a=1a = -1, b=6b = 6, and c=5c = -5. Plugging these values into the quadratic formula:

x=6±624(1)(5)2(1)=6±36202=6±162x = \frac{-6 \pm \sqrt{6^2 - 4(-1)(-5)}}{2(-1)} = \frac{-6 \pm \sqrt{36 - 20}}{-2} = \frac{-6 \pm \sqrt{16}}{-2}

x=6±42x = \frac{-6 \pm 4}{-2}

Thus, the two solutions are:

x=6+42=1andx=642=5x = \frac{-6 + 4}{-2} = 1 \quad \text{and} \quad x = \frac{-6 - 4}{-2} = 5

So the region is bounded by x=1x = 1 and x=5x = 5.

Step 2: Set up the volume integral

We will revolve the curve y=x2+6x5y = -x^2 + 6x - 5 around the x-axis. Using the disk method, the volume is:

V=π15[(x2+6x5)]2dxV = \pi \int_{1}^{5} \left[(-x^2 + 6x - 5)\right]^2 \, dx

Step 3: Compute the integral

We now need to expand and integrate the function:

(x2+6x5)2=(x412x3+46x260x+25)(-x^2 + 6x - 5)^2 = (x^4 - 12x^3 + 46x^2 - 60x + 25)

Thus, the volume integral becomes:

V=π15(x412x3+46x260x+25)dxV = \pi \int_{1}^{5} \left(x^4 - 12x^3 + 46x^2 - 60x + 25\right) \, dx

Now, integrate each 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, the integral becomes:

V=π[x5512x44+46x3360x22+25x]15V = \pi \left[\frac{x^5}{5} - 12 \cdot \frac{x^4}{4} + 46 \cdot \frac{x^3}{3} - 60 \cdot \frac{x^2}{2} + 25x \right]_{1}^{5}

Step 4: Evaluate the definite integral

Substitute x=5x = 5 and x=1x = 1 into the expression:

For x=5x = 5:

55512544+4653360522+255=6251875+1916.67750+125=41.67\frac{5^5}{5} - 12 \cdot \frac{5^4}{4} + 46 \cdot \frac{5^3}{3} - 60 \cdot \frac{5^2}{2} + 25 \cdot 5 = 625 - 1875 + 1916.67 - 750 + 125 = 41.67

For x=1x = 1:

15512144+4613360122+251=0.23+15.3330+25=7.53\frac{1^5}{5} - 12 \cdot \frac{1^4}{4} + 46 \cdot \frac{1^3}{3} - 60 \cdot \frac{1^2}{2} + 25 \cdot 1 = 0.2 - 3 + 15.33 - 30 + 25 = 7.53

Now, subtract the results:

41.677.53=34.1441.67 - 7.53 = 34.14

Step 5: Multiply by π\pi

Finally, multiply by π\pi:

V=π34.14107.2488V = \pi \cdot 34.14 \approx 107.2488

Final Answer:

The volume of the solid is approximately 107.2488\boxed{107.2488} cubic units.

Would you like further details or explanations?


Related Questions:

  1. How would the volume change if the region was revolved around the y-axis?
  2. What is the area of the region bounded by the curves before the revolution?
  3. How would the volume calculation change if the curve was revolved around the line y=1y = -1?
  4. Can you apply the washer method in this scenario? Why or why not?
  5. How do you determine the volume if the region is revolved around y=5y = 5 instead?

Tip: Always verify your limits of integration by checking where the curve intersects the axis of revolution.

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Math Problem Analysis

Mathematical Concepts

Calculus
Volume of Solids of Revolution
Integration

Formulas

Volume of a solid of revolution using the disk method: V = π ∫[a to b] [f(x)]^2 dx
Quadratic formula: x = [-b ± sqrt(b^2 - 4ac)] / 2a

Theorems

Disk Method for solids of revolution
Quadratic Formula for finding roots

Suitable Grade Level

Grades 11-12 (Advanced High School) or Early College