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Question
Math
Posted 8 months ago
Find the volume of the solid of revolution obtained by rotating the curve y=sin(x)y = sin(x), from x=0x = 0 to x=pix = pi, about the x-axis.
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Answer from Sia
Posted 8 months ago
Solution by Steps
step 1
To find the volume of the solid of revolution obtained by rotating the curve y=sin(x)y = \sin(x) from x=0x = 0 to x=πx = \pi about the x-axis, we use the disk method. The volume VV is given by the integral formula: V=π0π[sin(x)]2dxV = \pi \int_{0}^{\pi} [\sin(x)]^2 \, dx
step 2
We need to evaluate the integral 0πsin2(x)dx\int_{0}^{\pi} \sin^2(x) \, dx. Using the trigonometric identity sin2(x)=1cos(2x)2\sin^2(x) = \frac{1 - \cos(2x)}{2}, we rewrite the integral: 0πsin2(x)dx=0π1cos(2x)2dx\int_{0}^{\pi} \sin^2(x) \, dx = \int_{0}^{\pi} \frac{1 - \cos(2x)}{2} \, dx
step 3
Split the integral into two parts: 0π12dx0πcos(2x)2dx\int_{0}^{\pi} \frac{1}{2} \, dx - \int_{0}^{\pi} \frac{\cos(2x)}{2} \, dx
step 4
Evaluate the first part: 0π12dx=120π1dx=12[x]0π=π2\int_{0}^{\pi} \frac{1}{2} \, dx = \frac{1}{2} \int_{0}^{\pi} 1 \, dx = \frac{1}{2} [x]_{0}^{\pi} = \frac{\pi}{2}
step 5
Evaluate the second part: 0πcos(2x)2dx=120πcos(2x)dx\int_{0}^{\pi} \frac{\cos(2x)}{2} \, dx = \frac{1}{2} \int_{0}^{\pi} \cos(2x) \, dx
step 6
Use the substitution u=2xu = 2x, du=2dxdu = 2dx, so dx=du2dx = \frac{du}{2}. The limits change from x=0x = 0 to x=πx = \pi to u=0u = 0 to u=2πu = 2\pi. The integral becomes 1202πcos(u)du2=1402πcos(u)du\frac{1}{2} \int_{0}^{2\pi} \cos(u) \frac{du}{2} = \frac{1}{4} \int_{0}^{2\pi} \cos(u) \, du
step 7
Evaluate the integral: 02πcos(u)du=[sin(u)]02π=sin(2π)sin(0)=00=0\int_{0}^{2\pi} \cos(u) \, du = [\sin(u)]_{0}^{2\pi} = \sin(2\pi) - \sin(0) = 0 - 0 = 0
step 8
Combine the results: 0πsin2(x)dx=π20=π2\int_{0}^{\pi} \sin^2(x) \, dx = \frac{\pi}{2} - 0 = \frac{\pi}{2}
step 9
Multiply by π\pi to find the volume: V=π0πsin2(x)dx=ππ2=π22V = \pi \int_{0}^{\pi} \sin^2(x) \, dx = \pi \cdot \frac{\pi}{2} = \frac{\pi^2}{2}
Answer
The volume of the solid of revolution is π22\frac{\pi^2}{2}.
Key Concept
Disk Method
Explanation
The disk method involves integrating the square of the function being rotated around the axis, multiplied by π\pi, over the given interval.

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