Final answer:
The question appears to involve finding a general term for a sequence, but we're lacking sufficient details to identify the pattern or formula of the sequence. Without additional information on the nature of the sequence, we cannot provide the exact form of an.
Step-by-step explanation:
The student's question suggests that they are dealing with a sequence or series and they want to understand the general term, an, given specific terms of the sequence. To determine the pattern, we would ideally look at the difference between given terms or apply other methods to find a common relationship. Since there are gaps between the terms a9 and a20, this could be an arithmetic or geometric sequence, or neither, depending on the pattern of change. Unfortunately, the information provided in the question is incomplete, and we cannot deduce a specific pattern or formula for an without additional information about the sequence. Normally, if it were an arithmetic sequence, we would find the common difference and use it along with one of the given terms to find the formula. If it were a geometric sequence, we would find the common ratio. The correct approach would depend on understanding the specific pattern of the sequence.