- #1
MonkeyDLuffy
- 5
- 0
So I'm getting ready for an exam on tuesday, and I'm using each method for volumes of revolutions for every problem but I'm not getting the same answers. So, let's use this as an example:
y = 5x; the shaded region is from [1,2]
Using the disk method (about the x-axis) I find:
R(x) = 5x; r(x) = x
V = π ∫ 25x2 dx from [1,2] = 175π / 3
Using the shell method (about the x-axis) I find:
r(y) = y/5; h(y) = y
V = 2π ∫ (y2 / 5) dy from [5,10] = 350π / 3
I'd like to know why the shell method gave me a volume that is twice that of the one I found using the disk method.
y = 5x; the shaded region is from [1,2]
Using the disk method (about the x-axis) I find:
R(x) = 5x; r(x) = x
V = π ∫ 25x2 dx from [1,2] = 175π / 3
Using the shell method (about the x-axis) I find:
r(y) = y/5; h(y) = y
V = 2π ∫ (y2 / 5) dy from [5,10] = 350π / 3
I'd like to know why the shell method gave me a volume that is twice that of the one I found using the disk method.
Last edited: