I'm not quite sure what you are allowed to use here. If a diameter is given, then circumference = pi x diameter, or equivalently 2 x pi x radius.
And the area is pi x r^2, where r is the radius
But something tells me that's not what you meant.
Some other possibilities for your question:
1. area/circumference = r/2
2. circumference/area = 2/r
3. Ratios are fractions, and pi is a ratio between the circumference of a circle and its diameter.