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What could be the possible way to find the Nth term of following series where the sign toggles after each triangular number?

1 -2 -3 4 5 6 -7 -8 -9 -10 11 12 13 14 15 -16 -17 ....

The series cannot be in a Geometric Progression because there are 4 distinct triangular numbers in the above series.

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    Hint: the $n$-th term of your series is merely $(-1)^{f(n)} n$, for $n=1,2,\ldots$. You need to merely figure out what $f(n)$ is.2010-10-31
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    A _series_ is an infinite sum. Do you mean the $N$th term of the _sequence_, or do you want the $N$th partial sum of the series $1-2-3+4+5+6-7-\ldots$?2010-10-31
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    @Hans: He has explicitly mentioned about finding out the `Nth term` in his post.2010-10-31
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    @Prasoon: Of course, but if he doesn't intend to add the numbers, I find it a bit unnecessary to talk about terms in a series instead of just numbers in a sequence. Especially when he writes down a sequence and calls it a series. ;)2010-10-31

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