add/subt axis of rotation in volume problems?

[Parkstrailer]

I've seen many examples of volume of rotation problems off the y=0 and x=0 axis . . and I am totally confused as to whether add or subtract the axis of rotation when determining radius. For example (outer radius - inner radius), I found one site that said its always((axis of rotation - outer radius) - (aor - inner radius)) but then I've found sites that are adding it after?!?! So confused, please help! thx

 

[Deleted member 4993]

I've seen many examples of volume of rotation problems off the y=0 and x=0 axis . . and I am totally confused as to whether add or subtract the axis of rotation when determining radius. For example (outer radius - inner radius), I found one site that said its always((axis of rotation - outer radius) - (aor - inner radius)) but then I've found sites that are adding it after?!?! So confused, please help! thx

Put up two problems where you think you are getting confused. We will help you to sort it out.

I suggest do not try to get a rigid procedure in mind - be flexible. You should be able figure out the inner-radius & outer-radius from the particular situation presented in the problem.

 

[HallsofIvy]

The radius is a distance. And the distance form "A" to "B" is |A- B|. In other words, if A> B, A- B, if B> A, B- A. You NEVER add the two values to find a distance. You may be thinking of some thing like "the distance from -2 to 7 is 7- (-2)= 9 which "looks" like an addition but really is a subtraction.