The Shell method makes total sense, except for finding limits of integration.

How do you find them if rotating on the y axis (when you use the form y = "x variables here") or when rotating on the x axis (when you use the form x = "y variables here")

Here are two problems. How to find limits of integration?

1. region bounded by the curve $\displaystyle y = \sqrt{x}$ and the x axis, and the line x = 4 revolved around the y axis

2. region bounded by the curve $\displaystyle y = \sqrt{x}$, the x axis, and the line x = 4, revolved about the x axis.