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 Mejia May. in Malaysia
Jan 23,2025

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

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. **Determine the Points of Intersection:** The curves intersect where \( x^2 = 4 \), which gives \( x = \pm 2 \). So, the region of interest is between \( x = -2 \) and \( x = 2 \). 2. **Set Up the Washer Method:** The washer method involves integrating the area of washers (disks with holes) across the interval from \( x = -2 \) to \( x = 2 \). - **Outer Radius (\( R \)):** This is the distance from the line of rotation \( y = 5 \) to the curve \( y = x^2 \): \[ R = 5 - x^2 \] - **Inner Radius (\( r \)):** This is the distance from the line of rotation \( y = 5 \) to the horizontal line \( y = 4 \): \[ r = 5 - 4 = 1 \] - **Volume Integral:** The volume \( V \) is given by: \[ V = \pi \int_{-2}^{2} \left(R^2 - r^2\right) dx = \pi \int_{-2}^{2} \left((5 - x^2)^2 - 1^2\right) dx \] Expanding the integrand: \[ (5 - x^2)^2 - 1 = 25 - 10x^2 + x^4 - 1 = x^4 - 10x^2 + 24 \] So the integral becomes: \[ V = \pi \int_{-2}^{2} \left(x^4 - 10x^2 + 24\right) dx \] 3. **Evaluate the Integral:** Since the integrand is even, we can simplify the computation: \[ V = 2\pi \int_{0}^{2} \left(x^4 - 10x^2 + 24\right) dx \] Integrating term by term: \[ \int_{0}^{2} x^4 dx = \frac{2^5}{5} = \frac{32}{5}, \quad \int_{0}^{2} -10x^2 dx = -10 \cdot \frac{2^3}{3} = -\frac{80}{3}, \quad \int_{0}^{2} 24 dx = 24 \cdot 2 = 48 \] Combining these: \[ V = 2\pi \left(\frac{32}{5} - \frac{80}{3} + 48\right) = 2\pi \left(\frac{96}{15} - \frac{400}{15} + \frac{720}{15}\right) = 2\pi \cdot \frac{416}{15} = \frac{832\pi}{15} \] **Final Answer:** The volume is 832⁄15 multiplied by π. In symbols, Volume = (832 ⁄ 15) · π

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Bonus Knowledge

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, we start by determining the points of intersection of the curves to find the limits of integration. Setting \( x^2 = 4 \), we solve: \[ x^2 = 4 \implies x = -2 \quad \text{and} \quad x = 2. \] Thus, the region of interest is bounded between \( x = -2 \) and \( x = 2 \). When using the washer method, we identify the outer radius and the inner radius of the washers formed by the rotation around the line \( y = 5 \). - The outer radius \( R(x) \) is the distance from the line \( y = 5 \) to the line \( y = 4 \): \[ R(x) = 5 - 4 = 1. \] - The inner radius \( r(x) \) is the distance from the line \( y = 5 \) to the parabola \( y = x^2 \): \[ r(x) = 5 - x^2. \] The volume \( V \) of the solid of revolution is given by the integral of the difference of the squares of the outer and inner radii: \[ V = \pi \int_{-2}^{2} \left( R(x)^2 - r(x)^2 \right) \, dx. \] Now we compute: \[ R(x)^2 = 1^2 = 1, \] \[ r(x)^2 = (5 - x^2)^2 = 25 - 10x^2 + x^4. \] Thus, the integrand becomes: \[ R(x)^2 - r(x)^2 = 1 - (25 - 10x^2 + x^4) = 1 - 25 + 10x^2 - x^4 = -24 + 10x^2 - x^4. \] Now, we have: \[ V = \pi \int_{-2}^{2} (-24 + 10x^2 - x^4) \, dx. \] To evaluate the integral, we simplify: \[ V = \pi \left( \int_{-2}^{2} -24 \, dx + \int_{-2}^{2} 10x^2 \, dx - \int_{-2}^{2} x^4 \, dx \right). \] Calculating each integral separately: 1. \(\int_{-2}^{2} -24 \, dx = -24 \cdot (2 - (-2)) = -24 \cdot 4 = -96\). 2. \(\int_{-2}^{2} 10x^2 \, dx\): - The function \(10x^2\) is even, so: \[ 2 \cdot \int_{0}^{2} 10x^2 \, dx = 2 \cdot 10 \cdot \left[\frac{x^3}{3}\right]_{0}^{2} = 2 \cdot 10 \cdot \frac{8}{3} = \frac{160}{3}. \] 3. \(\int_{-2}^{2} x^4 \, dx\): - Again, using that it's even: \[ 2 \cdot \int_{0}^{2} x^4 \, dx = 2 \cdot \left[\frac{x^5}{5}\right]_{0}^{2} = 2 \cdot \frac{32}{5} = \frac{64}{5}. \] Putting it all together: \[ V = \pi \left(-96 + \frac{160}{3} - \frac{64}{5}\right). \] Now, we need to combine these fractions, finding a common denominator. The least common multiple of 3 and 5 is 15: \[ -96 = -\frac{1440}{15}, \] \[ \frac{160}{3} = \frac{800}{15}, \] \[ \frac{64}{5} = \frac{192}{15}. \] So now we combine: \[ V = \pi \left(-\frac{1440}{15} + \frac{800}{15} - \frac{192}{15}\right) = \pi \left(-\frac{1440 - 800 - 192}{15}\right) = \pi \

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