Using shell method, identify the radius and height of each shell and calculate the volume of the equation x2 + y2 = 2y revolved about y=4.
I understand the radius is =4-y, but because the bounded region is a circle, I don't follow how to calculate the height of each shell. I thought it would be ±√(2y - y2), one subtracted from the other, but the example shows the height is equal to 2√(4 - y2). I just don't follow how to get there.