not sure if this is the right section of the forum to put this but here goes anyhow:

I've got 2 circles of different radii, one inside the other, where the circumferences touch at a single point.

the centre point of the larger circle is within the edge of the inner circle.

I'm trying to come up with a formula that describes the change in area between the 2 circles for a complete rotation about the centre point of the smaller circle....any suggestions?