Series Question - Please help

ri2007 · Posted: Tue Nov 20, 2007 5:50 am Post subject: Series Question - Please help

Hi

Can any one help me understand what I am doing wrong. My exam is approaching real fast so would really apprecaite ur help. The link to the actual question is copied below

http://www.beatthegmat.com/ps-t6322.html

Here is what I did

The 5th term in the series is 1 (i.e 5-4)
the 6th term is 2 (i.e 6-4)
The 97th terms is 93

So essentially from the 5th term in the series to the 97th term we have a series of consequitive integers from 1 to 93

So the sum of all consequitive integers from 1 to 93 inclusive =

(94/2) * (94-1+1) = 4418

Now the sum of the first 4 numbers in the sequence is (2 + (-3) + 5 + (-1) ) = 3

So sum of first 97 terms is 4418 + 3 = 4421

This is obviously wrong, but I dont know why. Please Help.

Thanks in advance

samirpandeyit62 · Posted: Tue Nov 20, 2007 6:02 am Post subject:

this is repetitive series in which the a(n) = a(n-4)

i.e 5th term = first term & so on

now sum of 4 terms = 2-3+5-1 =3

we need to find the sum of first 97 terms

now we know the series repeats every 5th term

so we can say that the series has chunks of 4 with each having sum =3

(sum of 4 terms like the first 4)

so 97 will have 96 +1 terms 96/4 =24 & the 97th term will be same as first =2 (repetition)

so sum is 24*3 +2 =74

hope this clear.

ri2007 · Posted: Tue Nov 20, 2007 6:08 am Post subject:

thanks a lot samir