# Sequences and Series Help

1. Apr 24, 2007

### dtl42

This was the extra credit question on a quiz I had today, I am very anxious to find out the answer.

1. The problem statement, all variables and given/known data

Find the apparent Nth term of the sequence
2,-5,10,-17 ...... n

2. Relevant equations

Not sure really on this
an = .....

3. The attempt at a solution

(n)+(2n-1)*(-1)^(n-1)

The sequence starts with 2 and then increases by sequential odd numbers and alternates positive and negative.

Thanks very much

2. Apr 24, 2007

### G01

As far as I can tell, your answer is correct. Nice Job!

3. Apr 25, 2007

### dtl42

Actually I was thinking about it and my answer is only correct for the first two terms, after that it's in accurate, the real answer is

1+n^2 , I'm hoping that my teacher is swamped with grading and will only check the first two terms.

4. Apr 27, 2007

### sutupidmath

you forgot the (-1)^(n-1) part.
And your original answer does not wort even for the first two terms, just for the first one.

when u take n=2 you will get

2+(4-1)(-1)^1=2-3=-1

Last edited: Apr 27, 2007
5. Apr 27, 2007

### daveb

It looks to me like the nth term is the sum of the first n primes times (-1 raised to the power of n+1)

6. Apr 28, 2007

### dtl42

Im positive my answer works for the first two

(2+(4-1)) * (-1)^(2-1) = -5

7. Apr 28, 2007

### HallsofIvy

Staff Emeritus
But that's not what you wrote! In your first post you wrote
(n)+(2n-1)*(-1)^(n-1) where the (-1)^(n-1) is multiplied only by the second term, not the first.

In any case, "correct for 2 terms" is still not good enough. For the three terms given, I see"subtract 7, then add 5, then subtract seven, then add five, ..."