In this particular item, we are to derive an equation that would best represent the given data with regards to the diameters of certain circles.
From the data, it can be observed that the first term is 2.5 cm and that the difference of the first two terms is 0.6 cm. The difference of the succeeding terms is also 0.6 cm. This denotes that the given sequence is arithmetic with the first term equal to 2.5 cm and the common difference which is equal to 0.6 cm. Thus, given with the variables that are to be used in this item, the equation that would best represent the given is,
f(n) = 2.5 + (n - 1)(0.6)