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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 6.4.4

Look again at the region R in Figure 6.38 (p. 439). Explain why it would be difficult to use the washer method to find the volume of the solid of revolution that results when R is revolved about the y-axis.

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1
Identify the region R and the axis of revolution, which is the y-axis in this case.
Recall that the washer method involves slicing the solid perpendicular to the axis of revolution, creating washers (disks with holes) whose volume can be integrated.
When revolving around the y-axis, the slices should be horizontal (parallel to the x-axis) to form washers with radii expressed as functions of y.
If the region R is bounded by curves that are easier to express as functions of x rather than y, it becomes difficult to describe the inner and outer radii of the washers in terms of y, complicating the integral setup.
Therefore, the difficulty arises because the boundaries of R may not be easily invertible or expressible as functions of y, making the washer method cumbersome for this axis of revolution.

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Washer Method for Volumes of Revolution

The washer method involves slicing a solid perpendicular to the axis of revolution, creating washers (disks with holes). It requires expressing the outer and inner radii as functions of the variable of integration, typically aligned with the axis of revolution.
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Finding Volume Using Disks

Axis of Revolution and Variable of Integration

When revolving a region about the y-axis, the variable of integration is usually y. To use the washer method effectively, the boundaries of the region must be easily expressed as functions of y, which can be challenging if the region is defined primarily in terms of x.
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Disk Method Using y-Axis

Complexity of Expressing Radii in Terms of y

If the region R is bounded by curves given as functions of x, rewriting these boundaries as functions of y may be difficult or impossible to do explicitly. This complicates finding the inner and outer radii for washers, making the washer method less practical.
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