Infinite Series with Double Alternating Signs

• May 9th 2011, 05:13 PM
betzenderfer
Infinite Series with Double Alternating Signs
What is an example of an infinite series that has double alternating signs?? --++--++ or ++--++--

Something like: (-1)^(an expression of K)
• May 9th 2011, 05:23 PM
Ackbeet
$\sum_{k=0}^{\infty}\sin\left((2k+1)\,\frac{\pi}{4} \right)\,\frac{1}{x^{k}}?$