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 Post subject: Help with the nth term for 1, -1, 1, -1, ... pleasePosted: Tue Jan 06, 2009 4:52 pm

Joined: Sun Nov 30, 2008 6:38 pm
Posts: 19
My son has this for homework and I am stuck, please help. My method of finding the differences does not work.

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 Post subject: Posted: Tue Jan 06, 2009 5:03 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 14010
nth term = (-1) to the power of n+1

so when n is 1,3, 5 you get 1 (ie odd terms are 1)

and when n= 2, 4 you get -1 (even terms are negative)

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 Post subject: Posted: Tue Jan 06, 2009 6:37 pm

Joined: Sun Nov 30, 2008 6:38 pm
Posts: 19
Is there a formula for this, so far he has had work involving finding the first and second differences giving him either linear or quadratic equations. Your explanation makes perfect sense, I just wondered if there was a method that did not involve any intuition?

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 Post subject: Posted: Tue Jan 06, 2009 6:56 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 14010
No - when signs alternate like this there is usually a power involved - and that's where the difference method breaks down.

ie 2,4,8,16 is the powers of two - if you look at the differences you get 2,4,8 ... which is a bit of a clue.

I would expect that unless they have seen this 'trick' that few chilldren would get the question right so your child might get asked how they worked it out!! That's what I do when I set one like this

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 Post subject: Posted: Tue Jan 06, 2009 6:59 pm

Joined: Sun Nov 30, 2008 6:38 pm
Posts: 19
Thank you

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