Math Is Fun Forum

ox

Member

Registered: 2010-06-04

Posts: 3

Sequence problem

If I have a sequence that follows this pattern 5,6,9,14,21… where the first number in the sequence is 5. What is the 19th and 20th number in this sequence?

I don't get it.. anyone care to help?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Sequence problem

Hi ox;

329 and 366

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

ox

Member

Registered: 2010-06-04

Posts: 3

Re: Sequence problem

That works, thank you very much. For future reference do you mind explaining how you got the answer?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Sequence problem

We take your numbers:

5 , 6 , 9 , 14 , 21 ... We take the first differences.

  1   3    5     7 ... We take the 2nd differences.

     2   2   2     ... They are constant that means the underlying law for this sequence is a 2nd 
degree polynomial. Now just curve fit to interpolate the polynomial you need.

You will get y = x^2 - 2x + 6

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

ox

Member

Registered: 2010-06-04

Posts: 3

Re: Sequence problem

Oooh thank you. I feel as though I should have figured that one out myself. Maybe next time, you've been a great help.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Sequence problem

Hi ox;

Do you need to see a method to curve fit and get that on your own? And by the way, Welcome to MIF!

Wait where did he go? There is more!

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.