Method of Discs/Washers and Cylinderical Shells

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In summary, the conversation discusses three problems involving finding the volume of solids when rotating around a line that is not the x or y axis. The method used is cylindrical shells, and the conversation also includes a question about how to alter the formula when changing the variables. The key is to replace either x or y with x-a or y-a, depending on which variable is being changed.
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trogdor5
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There's just something I need cleared up. When rotating around a line that isn't the x or y axis,I'm not quite sure what to do. Here are some problems:

1) Find the volume of the solid that results when the region enclosed by y=√x, y=0, and x=9 is revolved around the line x=9.

2) Find the volume of the solid that results when the region enclosed by x=y² and x=y is revolved about the line y=-1

3) Use cylindrical shells to find the volume of the solid that is generated when the region that is enclosed by y=1/x^3 , x=1, x=2, y=0 is revolved about the line x=-1

I know that with the cylindrical shells the alteration is within the X part of the formula (2π∫x*f(x)dx) but I'm not exactly sure how to alternate the other two. For example, if the function is √x will the thing become √(x+1)^2 or [(√x)+1]^2? If you could just show me how to do each problem, that would solve my problems :) I know you guys aren't supposed to show how to do the problem, but the only way you can answer my questions is with showing mehow to do it :)

Thank you.
 
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Start by drawing a picture.
 
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Hi trogdor5! :smile:

(try using the X2 tag just above the Reply box :wink:)
trogdor5 said:
… For example, if the function is √x will the thing become √(x+1)^2 or [(√x)+1]^2?

If you're changing x, then you're replacing x by x - a, so y = √x becomes y = √(x - a).

If you're changing y, then you're replacing y by y - a, so y = √x becomes y - a = √x.
 

FAQ: Method of Discs/Washers and Cylinderical Shells

What is the Method of Discs/Washers and Cylindrical Shells?

The Method of Discs/Washers and Cylindrical Shells is a technique used in calculus to find the volume of a solid with a curved boundary. It involves breaking down the solid into discs or shells and using integration to find the total volume.

When is the Method of Discs/Washers and Cylindrical Shells used?

This method is typically used when the shape of the solid is difficult to determine or when the cross-sections of the solid are circles or semi-circles. It is commonly used in problems involving rotation of an area around an axis.

What is the difference between the Method of Discs/Washers and Cylindrical Shells?

The main difference between the two methods is the shape of the cross-sections used to calculate the volume. The Method of Discs/Washers uses discs or circles, while Cylindrical Shells uses cylinders or rectangles.

Is the Method of Discs/Washers and Cylindrical Shells accurate?

Yes, this method is very accurate when used correctly. However, it is important to make sure the cross-sections are perpendicular to the axis of rotation and that the limits of integration are set up correctly.

Can the Method of Discs/Washers and Cylindrical Shells be used for any shape?

No, the shape of the solid must have a circular or semi-circular cross-section in order for this method to be used. For other shapes, other methods such as the shell method or the cross-section method may be more appropriate.

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