Pregunta

Find the volume of the solid obtained by rotating the region bounded by the curves \( y = x^{2} \) and \( y = 4 \) about the line \( y = 5 \) using the washer method.

Ask by Norton Bob. in the United States
Jan 23,2025

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The volume of the solid is \( \frac{832\pi}{15} \).

Solución

To find the volume of the solid obtained by rotating the region bounded by the curves \( y = x^{2} \) and \( y = 4 \) about the line \( y = 5 \) using the washer method, follow these steps: ### 1. **Understand the Region and Rotation Axis** - The parabola \( y = x^2 \) intersects the horizontal line \( y = 4 \) at points \( x = \pm 2 \) because \( x^2 = 4 \) gives \( x = \pm 2 \). - The rotation is about the horizontal line \( y = 5 \), which lies above both \( y = x^2 \) and \( y = 4 \). ### 2. **Set Up the Washer Method Integral** - For each slice perpendicular to the x-axis between \( x = -2 \) and \( x = 2 \), calculate the outer and inner radii: - **Outer Radius (\( R \))**: Distance from the rotation axis \( y = 5 \) to the lower curve \( y = x^2 \): \[ R = 5 - x^2 \] - **Inner Radius (\( r \))**: Distance from the rotation axis \( y = 5 \) to the upper curve \( y = 4 \): \[ r = 5 - 4 = 1 \] - The area of a washer is \( \pi(R^2 - r^2) \). So, the volume integral is: \[ V = \pi \int_{-2}^{2} \left[ (5 - x^2)^2 - 1^2 \right] \, dx \] ### 3. **Simplify and Compute the Integral** - Expand the integrand: \[ (5 - x^2)^2 = 25 - 10x^2 + x^4 \] So, \[ V = \pi \int_{-2}^{2} \left( 25 - 10x^2 + x^4 - 1 \right) \, dx = \pi \int_{-2}^{2} \left( 24 - 10x^2 + x^4 \right) \, dx \] - Since the integrand is even, simplify by integrating from 0 to 2 and doubling the result: \[ V = 2\pi \int_{0}^{2} \left( 24 - 10x^2 + x^4 \right) \, dx \] - Evaluate the integral: \[ \int_{0}^{2} 24 \, dx = 24 \times 2 = 48 \] \[ \int_{0}^{2} 10x^2 \, dx = 10 \times \frac{2^3}{3} = \frac{80}{3} \] \[ \int_{0}^{2} x^4 \, dx = \frac{2^5}{5} = \frac{32}{5} \] - Combine the results: \[ V = 2\pi \left( 48 - \frac{80}{3} + \frac{32}{5} \right) \] Convert to a common denominator (15): \[ 48 = \frac{720}{15}, \quad \frac{80}{3} = \frac{400}{15}, \quad \frac{32}{5} = \frac{96}{15} \] \[ V = 2\pi \left( \frac{720 - 400 + 96}{15} \right) = 2\pi \left( \frac{416}{15} \right) = \frac{832\pi}{15} \] ### **Final Answer** The volume is \( \dfrac{832}{15} \) multiplied by π. Thus, \[ V = \frac{832\,\pi}{15} \]

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To find the volume of the solid obtained by rotating the region bounded by the curves \( y = x^{2} \) and \( y = 4 \) around the line \( y = 5 \), we will use the washer method. First, we need to determine the points of intersection between the curves \( y = x^2 \) and \( y = 4 \). Setting them equal gives: \[ x^2 = 4 \implies x = \pm 2. \] Thus, the region of integration is bounded by \( x = -2 \) and \( x = 2 \). Next, we identify the outer and inner radii of the washers. The outer radius \( R(x) \) is the distance from the line \( y = 5 \) to the curve \( y = 4 \): \[ R(x) = 5 - 4 = 1. \] The inner radius \( r(x) \) is the distance from the line \( y = 5 \) to the curve \( y = x^2 \): \[ r(x) = 5 - x^2. \] Using the washer method, we can express the volume \( V \) as: \[ V = \pi \int_{-2}^{2} \left[ R(x)^2 - r(x)^2 \right] \, dx = \pi \int_{-2}^{2} \left[ 1^2 - (5 - x^2)^2 \right] \, dx. \] Now we simplify the expression: \[ V = \pi \int_{-2}^{2} \left[ 1 - (5 - x^2)^2 \right] \, dx. \] Calculating \( (5 - x^2)^2 \): \[ (5 - x^2)^2 = 25 - 10x^2 + x^4. \] Thus, the integral becomes: \[ V = \pi \int_{-2}^{2} \left[ 1 - (25 - 10x^2 + x^4) \right] \, dx = \pi \int_{-2}^{2} \left[-24 + 10x^2 - x^4\right] \, dx. \] Now we break it down: \[ V = \pi \left[ \int_{-2}^{2} -24 \, dx + \int_{-2}^{2} 10x^2 \, dx - \int_{-2}^{2} x^4 \, dx \right]. \] Calculating each integral: 1. \(\int_{-2}^{2} -24 \, dx = -24 \times (2 - (-2)) = -24 \times 4 = -96\). 2. For \(\int_{-2}^{2} 10x^2 \, dx\): \[ = 10 \times \left[ \frac{x^3}{3} \right]_{-2}^{2} = 10 \times \left[\frac{2^3}{3} - \frac{(-2)^3}{3}\right] = 10 \times \left[\frac{8}{3} + \frac{8}{3}\right] = 10 \times \frac{16}{3} = \frac{160}{3}. \] 3. For \(\int_{-2}^{2} x^4 \, dx\): \[ = \left[ \frac{x^5}{5} \right]_{-2}^{2} = \frac{2^5}{5} - \frac{(-2)^5}{5} = \frac{32}{5} + \frac{32}{5} = \frac{64}{5}. \] Now substituting these back: \[ V = \pi \left( -96 + \frac{160}{3} - \frac{64}{5} \right). \] To combine these, we first find a common denominator (which is 15): \[ -96 = -\frac{1440}{15}, \quad \frac{160}{3} = \frac{800}{15}, \quad \frac{64}{5} = \frac{192}{15}. \] Thus: \[ V = \pi \left( -\frac{1440}{15} + \frac{800}{15} - \frac{192}{15} \right) = \pi \left( \frac{-1440 + 800 - 192}{15} \right) = \pi \left( \frac{-832}{15} \right) = -\frac{832\pi}{15

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