- #1

Rectifier

Gold Member

- 313

- 4

**The problem**

I want to calculate the following sum

$$ \sum^{5}_{k=2} \frac{k(-1)^k}{2^k} $$

The attempt

The attempt

I wrote ## \frac{(-1)^k}{2^k} ## as ##\frac{1}{(-2)^k}##. I was hoping that I could calculate the sum ## \sum^{5}_{k=2} \frac{k(-1)^k}{2^k} ## by multiplying the sums ##\sum^{5}_{k=2} k## and ## \sum^{5}_{k=2} \frac{1}{(-2)^k}##, but I was wrong (wolfram).