quick question...in using the circular ring method (a.k.a. washer) the dy or dx depends on the axis of revolution right? i mean i use dy if the axis of revolution is the y-axis and i use dx if the axis of revolution is the x-axis?

Question

kinggeorge · Answer

Correct.

lgbasallote · Answer

same  with circular disk right?

kinggeorge · Answer

Yup.

lgbasallote · Answer

and cylindrical shell is opposite?

kinggeorge · Answer

I don't think so. Give me a minute just to make sure though.

lgbasallote · Answer

hmm okay

kinggeorge · Answer

Wait, nevermind. It is opposite. I got confused as to what the cylindrical method was.

lgbasallote · Answer

hahaha =))) well one more thing i ALWAYS  get confused...the limits o.O the limits of circular dis is based on the d? i.e. if it's dx then the upper limit and lower limit are on the x axis?

kinggeorge · Answer

Yes. dx means the limits are x-values, and dy means the limits are y-values.

lgbasallote · Answer

this applies to ALL three methods?

kinggeorge · Answer

that applies to all the methods.

lgbasallote · Answer

hmm  that seems like all that i get confused about...except the radius for cylindrical shell...any methods for that?

kinggeorge · Answer

What do you mean?

lgbasallote · Answer

$$\int_a^b 2\pi r h dx$$

that's the formula for cylindrical shell right where h is the given function...well that i get...but the r is always where i get confused o.O amistre said that to find radius it was x  - axis of revolution...but then dpalnc said it was axis of revolution - x

lgbasallote · Answer

or maybe it's x - axis of revolution when the axis of revolution is to the right of the function and it's axis of revolution - x when the axis is to the left of x

then y - axis of revolution when axis is below the graph and axis - y when axis is above the graph? yes?

kinggeorge · Answer

I'm not actually sure about that either. It's been a while since I've done these.

lgbasallote · Answer

oh i see...ill just try them calculus experts..thanks :DD