a(r)=ðr²

This is a function. The right hand side is the rule. r is the independent variable. The domain of r (i.e. the values it can assume) is ℝ+, meaning all positive real values, since we do not have a negative radius.

Does This Have an Inverse?

From algebra, do you know how to find the inverse?

The same method is used.

Have you tried to find the inverse?