What Is the Nth Term of the Sequence 2, -5, 10, -17?

[dtl42]

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


1. Homework Statement

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



2. Homework 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

 

[G01]

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

 

[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.

 

[sutupidmath]

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.

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

 

[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)

 

[dtl42]

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

Im positive my answer works for the first two

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

 

[HallsofIvy]

Im positive my answer works for the first two

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

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, ..."