# Thread: Infinite Series with Double Alternating Signs

1. ## Infinite Series with Double Alternating Signs

What is an example of an infinite series that has double alternating signs?? --++--++ or ++--++--
I'm drawing a blank! Please help!

Something like: (-1)^(an expression of K)

2. How about

$\displaystyle \sum_{k=0}^{\infty}\sin\left((2k+1)\,\frac{\pi}{4} \right)\,\frac{1}{x^{k}}?$