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Ch. 6 - Applications of Integration
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 6, Problema 6.4.46

45–48. Shell and washer methods about other lines Use both the shell method and the washer method to find the volume of the solid that is generated when the region in the first quadrant bounded by y = x²,y=1, and x=0 is revolved about the following lines. 


x = -1

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1
First, identify the region bounded by the curves: \(y = x^{2}\), \(y = 1\), and \(x = 0\) in the first quadrant. This region lies between \(x=0\) and \(x=1\) because when \(y=1\), \(x = \sqrt{1} = 1\).
For the shell method about the line \(x = -1\), consider vertical slices parallel to the axis of rotation. The radius of a shell at position \(x\) is the distance from \(x\) to \(-1\), which is \(r = x - (-1) = x + 1\). The height of the shell is the vertical distance between \(y=1\) and \(y=x^{2}\), so \(h = 1 - x^{2}\).
Set up the shell method integral for the volume \(V\) as: \(V = 2\pi \int_{0}^{1} (\text{radius})(\text{height}) \, dx = 2\pi \int_{0}^{1} (x + 1)(1 - x^{2}) \, dx\).
For the washer method about the line \(x = -1\), consider horizontal slices perpendicular to the axis of rotation. Express \(x\) in terms of \(y\): from \(y = x^{2}\), we get \(x = \sqrt{y}\). The region extends from \(y=0\) to \(y=1\).
The outer radius \(R\) is the distance from \(x = -1\) to the right boundary \(x=1\), so \(R = 1 - (-1) = 2\). The inner radius \(r\) is the distance from \(x = -1\) to the curve \(x = \sqrt{y}\), so \(r = \sqrt{y} - (-1) = \sqrt{y} + 1\). Set up the washer method integral for the volume \(V\) as: \(V = \pi \int_{0}^{1} \left(R^{2} - r^{2}\right) dy = \pi \int_{0}^{1} \left(2^{2} - (\sqrt{y} + 1)^{2}\right) dy\).

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Shell Method for Volume

The shell method calculates the volume of a solid of revolution by integrating cylindrical shells. Each shell's volume is found by multiplying its circumference, height, and thickness. This method is especially useful when the axis of rotation is parallel to the axis of the function being integrated.
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Washer Method for Volume

The washer method involves slicing the solid perpendicular to the axis of rotation, creating washers (disks with holes). The volume is found by integrating the difference between the outer and inner radii squared, times π, over the interval. This method is ideal when the solid has a hollow center.
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Finding Volume Using Disks

Adjusting for Axis of Rotation Not on Coordinate Axes

When revolving around lines other than the coordinate axes, such as x = -1, distances (radii) must be measured relative to that line. This requires shifting the radius expressions by the distance from the curve to the axis of rotation, ensuring accurate calculation of shell heights or washer radii.
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