Question

Calculate the volume of the solid formed by rotating the region bounded by the curve \( y = x^2 \) and the line \( y = 1 \) around the x-axis using the washer method.

Ask by Schwartz Carroll. in Nigeria
Feb 25,2025

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Tutor-Verified Answer

Answer

The volume of the solid is \( \frac{8\pi}{5} \) cubic units.

Solution

To solve the problem using the washer method, follow these steps: 1. Identify the region bounded by the curves: • The curve: y = x² • The line: y = 1 2. Find the intersection points of the curves by setting x² = 1:   x² = 1 ⟹ x = ±1 3. When you rotate the region about the x-axis, each vertical slice at a position x becomes a washer (a disk with a hole). The outer radius of the washer is the distance from the x-axis to the line y = 1, and the inner radius is the distance from the x-axis to the curve y = x². • Outer radius, R: The distance from the x-axis to y = 1 is constant, so R = 1. • Inner radius, r: The distance from the x-axis to y = x² is r = x². 4. The area A(x) of a washer is given by:   A(x) = π[R² - r²]   A(x) = π[1² - (x²)²] = π[1 - x⁴] 5. To get the volume, integrate A(x) with respect to x between the limits x = -1 and x = 1:   V = ∫₋₁¹ A(x) dx = π∫₋₁¹ (1 - x⁴) dx 6. Notice that the integrand (1 - x⁴) is an even function, since x⁴ is even. Therefore, you can simplify the integration by evaluating it from 0 to 1 and doubling the result:   V = 2π∫₀¹ (1 - x⁴) dx 7. Compute the integral:   ∫₀¹ (1 - x⁴) dx = [x - (x⁵/5)] evaluated from 0 to 1   = (1 - 1/5) - (0 - 0) = 1 - 1/5 = 4/5 8. Multiply by 2π:   V = 2π * (4/5) = 8π/5 Thus, the volume of the solid formed by rotating the region about the x-axis is (8π)/5 cubic units.

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The Deep Dive

To find the volume of the solid formed by rotating the region bounded by the curve \( y = x^2 \) and the line \( y = 1 \) around the x-axis using the washer method, we first need to determine the points of intersection between the curve and the line. Setting \( x^2 = 1 \), we have: \[ x^2 = 1 \implies x = \pm 1 \] This indicates that the region of interest in the x-axis is from \( x = -1 \) to \( x = 1 \). The formula for the volume \( V \) of a solid of revolution using the washer method is given by: \[ V = \pi \int_a^b \left( R(x)^2 - r(x)^2 \right) dx \] where \( R(x) \) is the outer radius and \( r(x) \) is the inner radius. In this case: - The outer radius \( R(x) \) is the line \( y = 1 \). - The inner radius \( r(x) \) is the curve \( y = x^2 \). Thus, we have: \[ R(x) = 1, \quad r(x) = x^2 \] Now we substitute these into the volume formula: \[ V = \pi \int_{-1}^{1} \left( (1)^2 - (x^2)^2 \right) dx \] This simplifies to: \[ V = \pi \int_{-1}^{1} \left( 1 - x^4 \right) dx \] Now we can compute the integral: \[ V = \pi \left[ \int_{-1}^{1} 1 \, dx - \int_{-1}^{1} x^4 \, dx \right] \] Calculating the first integral: \[ \int_{-1}^{1} 1 \, dx = 2 \] Now calculating the second integral: \[ \int_{-1}^{1} x^4 \, dx = 2 \int_{0}^{1} x^4 \, dx = 2 \left[ \frac{x^5}{5} \right]_{0}^{1} = 2 \cdot \frac{1}{5} = \frac{2}{5} \] Putting everything together: \[ V = \pi \left( 2 - \frac{2}{5} \right) = \pi \left( \frac{10}{5} - \frac{2}{5} \right) = \pi \left( \frac{8}{5} \right) \] Thus, the final volume of the solid is: \[ V = \frac{8\pi}{5} \]

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