Can any one help out

I'm afraid Ive never used that formula you have mentioned, it doesnt seem to work either

1 + 2 + 3 = 6

Using your formula for the sum, it would be (3-1) + 2 = 4

In general, the sum of a series of numbers starting from 1 is given by n(n+1)/2, where n is the number of terms.

For an even more general case, i.e. an arithmetic progression, sum is given by (a1 + an)*n/2, where n is the number of terms, a1 is the first term and an is the last term of the series. Remember, this works only with arithmetic progression i.e. where the difference between the terms is the same (e.g. 2 4 6 8 10....)

I have never seen any AP based questions on the GMAT though. If you have, please post them here.

Thanks
Kunal

Hey guys,

It looks like the aforementioned formula has been edited out, but I still want to point out that quote:

I'm afraid Ive never used that formula you have mentioned, it doesnt seem to work either

One thing I love about the GMAT is that formulas are very seldomly required, and it may not pay to memorize formulas when you can likely find your own way.

My first thought on this one is that if negative values are included, you can cancel them with their positive counterparts:

-15 + 15 = 0
-14 + 14 = 0
and so on

So, really, this sum would just be 16-42, inclusive, since the -15 through 15 set sums to zero.

Another thought if the problem were somehow worded differently - you could always sum the negative set and the positive set separately and then subtract the negative from the positive. [/quote]