SOLUTION: What is the sequence here? 4,9,19,39,79. Find the next term.

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Question 5391: What is the sequence here? 4,9,19,39,79. Find the next term.
Found 3 solutions by khwang, Abbey, longjonsilver:
Answer by khwang(438) About Me  (Show Source):
You can put this solution on YOUR website!
4,9,19,39,79
Let sequence be an, we have
a1 =4, an= 2*an(n-1)+1, for n>=2
Hence, the next term a6 = 2 a5 + 1 = 159
Kenny

Answer by Abbey(339) About Me  (Show Source):
You can put this solution on YOUR website!
The difference between 4 and 9 is 5
The difference between 9 and 19 is 10
The difference between 19 and 39 is 20
The difference between 39 and 79 is 40

So it looks as if the difference is being doubled each time, so the next answer will have a difference of 2*40, or 80
79+80=159

Answer by longjonsilver(2297) About Me  (Show Source):
You can put this solution on YOUR website!
4,9,19,39,79
the thing is with these...spot a pattern of sorts. Now 4,9,19,39,79 means nothing, but what about 5, 10, 20, 50, 80? That is much more recognisable as a pattern...
n = 1, 2, 3, 4, 5
--> 5x1, +++5x2, +++5x4, +++5x8, +++5x16
--> 5x2%5E0, 5x2%5E1, 5x2%5E2, 5x2%5E3, 5x2%5E4

--> 5x2%5E%28n-1%29

Now the original question is 1 less than each of these terms, so final answer is 5x2%5E%28n-1%29+-+1
jon.