Test analysis: if the number of triangles constitutes a series, then the general term and the number of squares constitute a series, then the general term. If the four options are brought into the formulas of two general terms in turn, we can know that only 1225 conforms to two series.

Comments: To find the general term of the first few items of a series, we must first grasp the characteristics of the first few items, find out the relationship between the number n and the item, or find the recursive formula through the relationship between two adjacent items, and then get the general term formula.