SOLUTION: 5,5,6,8,11,15 what is the 20th term and 21th

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Question 512608: 5,5,6,8,11,15 what is the 20th term and 21th
Answer by Edwin McCravy(20060)   (Show Source): You can put this solution on YOUR website!
          1st          2nd  
n  an  differences differences
1   5     0           1               
2   5     1           1
3   6     2           1  
4   8     3           1  
5  11     4
6  15     

Since the 2nd differences are all constant we try a 2nd
degree polynomial in n





Simplifying:



Solve that system and get A = , B = , C = 6

So the equation



becomes:







Substituting n = 20









Substituting n = 21










Answers: , 

Edwin
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