SOLUTION: How do you put the numbers 3,8,18,38,78 in arithmetic formula?

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Question 990551: How do you put the numbers 3,8,18,38,78 in arithmetic formula?
Answer by rothauserc(4718) About Me  (Show Source):
You can put this solution on YOUR website!
3, 8, 18, 38, 78
begin by calculating the difference between the numbers in the given arithmetic sequence
5, 10, 20, 40
note that these are multiples of 5, therefore
Xn = Xn-1 + 5*(2^(n-2))