There are many possible answers.
If the sequence is generated by the quartic polynomial:
t(n) = (5n4 - 38n3 + 127n2 - 166n + 96)/24 for n = 1, 2, 3, ...
the next number will be 81
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34 55 89 etc..
34 because 1+1 is 2, 1+2 is is 3, 2+3 is 5, 3+5 is 8, so on and so on. to get the next number, you add the 2 preceding numbers. so 13+21 is 34
89.
If you mean: 1 1 2 3 5 8 13 21 then the next number is 13+21 = 34
13 21 34 55