Dear All
Can you please let me know how to calculate the volume obtained by rotating the y=f(x) curve about the xaxis for x=x1 to x2?
Can you please let me have the answer for rotating it about the yaxis for y=y1 to y2 as well ?
Thank you
volume?

 Posts: 9
 Joined: Sat Jan 01, 2011 5:35 pm
Re: volume?
The easiest way to do this is through the washer method.
$$V = \pi\int_{x_1}^{x_2}(f(x)^2)dx$$
This works for rotation along the xaxis. Intuitively, you're adding up loads of infinitesimally small washers of area $$\pi f(x)^2.$$
If the function is being rotated around the y axis, you'll need to get the curve as a function of y instead, as we'll have a dy integral, and then apply the exact same procedure.
alternatively, you can use the cylinder method.
$$V=2\pi\int_{x_1}^{x_2}x {f(x)} dx$$
$$V = \pi\int_{x_1}^{x_2}(f(x)^2)dx$$
This works for rotation along the xaxis. Intuitively, you're adding up loads of infinitesimally small washers of area $$\pi f(x)^2.$$
If the function is being rotated around the y axis, you'll need to get the curve as a function of y instead, as we'll have a dy integral, and then apply the exact same procedure.
alternatively, you can use the cylinder method.
$$V=2\pi\int_{x_1}^{x_2}x {f(x)} dx$$