Set up, but do not evaluate, an integral for the volume of the solid obtained by rotating the region bounded by the given curves about the specified line.y = ln(x), y = 0, x = 4; about the x-axis
Question
Set up, but do not evaluate, an integral for the volume of the solid obtained by rotating the region bounded by the given curves about the specified line.y = ln(x), y = 0, x = 4; about the x-axis
Solution
To set up the integral for the volume of the solid obtained by rotating the region bounded by the given curves about the x-axis, we use the disk method. The disk method formula is V = π ∫ [R(x)]² dx from a to b, where R(x) is the radius of the disk at x.
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Identify the curves: The curves are y = ln(x) and y = 0 (which is the x-axis).
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Identify the region: The region bounded by these curves is the area above the x-axis (y = 0), below the curve y = ln(x), and to the left of the line x = 4.
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Identify the radius: When we rotate this region about the x-axis, the radius of the resulting disk at a given x is the y-coordinate of the curve y = ln(x). So, R(x) = ln(x).
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Identify the limits of integration: We are integrating along the x-axis, from x = 1 (where y = ln(x) intersects the x-axis) to x = 4.
So, the integral to represent the volume of the solid is:
V = π ∫ from 1 to 4 [ln(x)]² dx
This integral is not easy to evaluate by elementary methods, but it can be computed using numerical methods or software.
Similar Questions
Use the method of cylindrical shells to find the volume of the solid obtained by rotating the region bounded by the given curves about the x-axis.x = 3 + (y − 4)2, x = 4
Use the method of cylindrical shells to find the volume V generated by rotating the region bounded by the given curves about the y-axis.y = 4e−x2, y = 0, x = 0, x = 1Sketch the region and a typical shell.Step 1Rotating a vertical strip around the y-axis creates a cylinder with radius r = $$x and height h = $$4e−x2 . Sketch the region and a typical shell. Step 2Now we can say that the volume of the solid created by rotating the region under y = 4e−x2and above the x-axis between x = 0 and x = 1 around the y-axis isV = b2𝜋rh dxa= 2𝜋x dx.
Step 2Now we can say that the volume of the solid created by rotating the region under y = 4e−x2and above the x-axis between x = 0 and x = 1 around the y-axis isV = b2𝜋rh dxa= 1 1 2𝜋x $$4e−x2 dx.0Step 3The integral 2𝜋 4xe−x2 dx can be done with the substitution u = $$−x2 and du = $$−2x dx.Step 4With the substitution, we have 2𝜋 4xe−x2 dx = − $$4π eu du = − $$4πeu + C.Step 5Going back to x, the volume of our solid is
Now we can say that the volume of the solid created by rotating the region under y = 4e−x2and above the x-axis between x = 0 and x = 1 around the y-axis isV = b2𝜋rh dxa= 2𝜋x
Determine the volume of the solid obtained by rotating the region bounded by𝑦 = 𝑥2 − 4x + 5, x = 1, x = 4 and the x-axis about the x-axis. (78𝜋5
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