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# Sequences help!!!

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1. Can someone explain how to do this please?
By writing down the first terms or otherwise, find the recurrence formula that defines the following sequences:
Un = (-1)^n n
2. (Original post by Purpleunicorn197)
Can someone explain how to do this please?
By writing down the first terms or otherwise, find the recurrence formula that defines the following sequences:
Un = (-1)^n n
is that

????
3. (Original post by dskinner)
is that

????
yes
4. (Original post by Purpleunicorn197)
yes
Well finding the first term is easy, let n be any number.

put in 1
you get -1 out

put 2
you get -1 out

put in 64378268427916547924789136579824 758931
you still get -1 out

but i don't quite understand the second part :/

Zacken what would you do for the second bit?
5. (Original post by Purpleunicorn197)
yes
Write out the sequence. You can see that , , , , etc...

i.e: to go from the first term to the second term, you need to multiply the result by -1 and then either add 1. To go from the second to the third, you need to multiply by -1 and then subtract 1.

So you should be thinking but you need to find a way to write the such that it adds one when finding the second term (i.e: even cases) and subtracts one when finding odd terms. Think about it in terms of .

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