Answer:
26
Explanation:
The difference between 1 and 4 is 3
The difference between 4 and 8 is 4
The difference between 8 and 13 is 5
The difference between 13 and 19 is 6
The difference between 19 and 26 is 7
a_n = 1/2 (n^2 + 3 n - 2) (for all terms given)
1, 4, 8, 13, 19, 26, 34, 43, 53, 64, 76, 89, 103, 118, 134, 151, 169, 188, 208, 229, ...
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