# Homework Help: Determining best method for volumes?

1. Mar 23, 2007

### dnt

1. The problem statement, all variables and given/known data

No specific question but what is the best way to determine which method is the best for solving for volumes (using integration) of shapes as they revolve around axis/lines. disk method? washer method? shell method? other?

what exactly do you look for that may hint towards one method over the other?

2. Relevant equations

n/a

3. The attempt at a solution

2. Mar 24, 2007

### Nibbler

It depends on what axis they're revolving.

3. Mar 24, 2007

### HallsofIvy

Which is better depends not only about which axis they are revolving but also the specific form of the functions describing the boundaries. About the only thing one can say in general is to consider each and decide which gives the simplest integral.

4. Mar 24, 2007

### dnt

well you only have two axis, right? if its x which is best? if its y which is best?

5. Mar 24, 2007

### mjsd

to do volume with calculus: whether you chop the object up into thin disks or thin shells depends highly on the shape of the object. eg: consider the area under the graph e^(-x) and x in [1,2], if vol. is formed by revolving this area about x axis, then disk method is the natural choice, whereas if revolved about y axis, then shell method would be better.

6. Mar 25, 2007

### dnt

why? what made you pick one over the other? what specifics do you look for that would cause you to lean towards one method over the other?

7. Mar 25, 2007

### Nibbler

Because one of the methods is easier than the other. In certain situations, the shell method is easier to do than the disk method.

And we all know mathematicians are lazy. ;)

Think about it this way: if you were to find the area of a cone (y = x) that revolved around the x-axis, you'ld use the disk method. Not saying that you can't use the shell method, but its just easier to use disk.