# Washer/Shell volumes

1. Dec 14, 2006

### CalcNoob

Hi there,

I'm new here and I'm really glad that I found a community to discuss calculus even though its not really my favorite course (although I enjoy some of its topics).

My first question here is about evaluating volumes of shapes that have no solid interiors. I know how to use the washer formula, my problem is determining the outer function and the inner function. I know I'm supposed to subtract the inner shape volume from the outer one. But in some problems, they give us the functions in which the region is enclosed by (e.g. f(x) and g(x), y=a,y=b )but they don't provide any graph for it. How am I supposed to know if f(x) > g(x) for all [a,b] or vice-verse ?
Usually, in the exams they give us known functions but with modifications , such as y=sqrt(x^2-25), I think such functions are easier to graph it. Thanks and advance, and sorry for my bad english , hopefully you understood my question :-)

Regards,

2. Dec 16, 2006

### Gib Z

I'm not sure it will matter very much, if you get a negative value by using choose the wrong functions for inner/outter, then just absolute value it. Same problem as not knowing wether a or b is bigger to choose a-b or b-a, absolute valuing it will give you the difference anyway, which is what you want, The difference between the outer volume and inner volume. Hope i helped.