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Please, can anyone help me get the general formula of the sequence below.

when n=3 the number of arrows is 2
when n=4 the number of arrows is 4
when n=5 the number of arrows is 8
when n=6 the number of arrows is 12
when n=7 the number of arrows is 18
when n=8 the number of arrows is 24
when n=9 the number of arrows is 32 
when n=10 the number of arrows is 40 etc

the n start from 3 upward.

Thank you

babs
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1 Answers1

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I'm not seeing an immediate way to combine it into a single equation, but it looks like if you split it between odd and evens there are two basic formulas. Maybe this will help you get on the right track:

Even: (n^2/2) - n Odd: [(n^2 + 1)/2] - n

Edit

I got the idea for the combined formula from this Stack Exchange https://meta.stackexchange.com/questions/76902/how-can-i-write-math-formula-in-a-post

Here is the formula:

enter image description here

Note the absolute value around the (-1)^n - 1. If you don't have that, it will give you the -1 instead of +1 for every odd value.

Community
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Hopeless
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  • with your formula, I was able to get the correct answer. Thanks for the clue – babs Jul 09 '16 at 13:07
  • @babs Where you able to combine it into a single formula, or did the answer require two separate definitions? – Hopeless Jul 11 '16 at 20:35
  • It was two separate formulas. Combining it to form one seems not to work so I have to loop. Do you have a combine formula for it? – babs Jul 12 '16 at 15:50
  • @babs I updated my answer with a combining formula. If it works for you can you mark my answer as correct? – Hopeless Jul 13 '16 at 00:06
  • pls can you rewrite the formula for me. I didn't get your formula properly. Though, I understood the modulus aspect. – babs Jul 13 '16 at 14:07
  • @babs I was able to upload as an image. Let me know if that works for you. – Hopeless Jul 13 '16 at 20:42