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Answer:
[tex]a_n=\dfrac{1}{2}n^3-\dfrac{1}{2}n^2+6n-3[/tex]
Step-by-step explanation:
The first differences are 8, 13, 21. The second differences are 5, 8. The second differences are not constant, so this is not a quadratic sequence.
If we assume it is a cubic sequence, a calculator tells us the formula is ...
[tex]a_n=\dfrac{1}{2}n^3-\dfrac{1}{2}n^2+6n-3[/tex]
