Question

What does the u stand for in these problems? is it the same as r?

Answer 1

I think "u" stands for units. $u^2$ means units squared which is like saying ft^2 if you were looking for the area of something with feet dimensions. So in this case, the circle has a diameter of x units (u) and an area of x units^2 ($u^2$). It is not the same as r. "r" is most always going to be used to represent the radius.