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I don't think theres any straightforward formulas to use for sequences and series' in finding a formula.

My attempt involved finding two numbers that add to 13 and subtract to 3 which is 8 and 5, but what I'm stuck on is inserting the n variable so that the pattern will keep repeating 13,3,13,3..so on. Totally lost on how to do it and it doesn't even seem that hard.