Originally Posted by

**Istafa** Thats precisely correct

Think about it this way:

You are rotating two of the same area 360 degrees about the axis, this will give you twice the volume

You need only half the total area to be rotated around for the solid required

Sketch the graph, labelling co-ordinates. It helps to visualise and to switch between the axis with dy or dx more easily!!

The integration also looks quite tricky, you probably need extra help on this. I'm not exaclty sure how to integrate arcos^2(y)???