# Math Help - Cylindrical shells method:

1. ## Cylindrical shells method:

The region bounded by the line y = 1 and the parabola x^2 = 4y
is revolved about the line y = 1.....
Take the rectangular elements of area parallel to the axis of revolution

I set :
height = $2\sqrt{y} - (-2\sqrt{y})$

$2\pi \int_{-2}^{2} (2\sqrt{y} - (-2\sqrt{y})(1 - y)dy$

$2\pi \int_{-2}^{2} (4\sqrt{y})(1 - y)dy$

i need help
my answer seems to be undefined...

2. The limits of your integration should be y=0 and y=1.

You are using dy, not dx.
dy goes from y=0 to y=1.