# Homework Help: Test for convergence of the series

1. Feb 26, 2012

### DarkStalker

1. The problem statement, all variables and given/known data
Q)
Summation from 1 to infinity
(1+(-1)^i) / (8i+2^i)

This series apparently converges and I can't figure out why.

2. Relevant equations

3. The attempt at a solution

(1+(-1)^i) / i(8+2^i/i)

Taking the absolute value of the above generalization:

2/i(8+2^i/i)

Rearranging that would give:

2/i * (1/(8+2^i/i)

Now I thought that since 2/i would diverge, the entire series should diverge.

Comparing it to the geometric series (1/2^i) implies converges, but I don't know what's wrong with the above

2. Feb 26, 2012

### vela

Staff Emeritus
I don't understand your reasoning here. It's like saying
$$\sum_{n=1}^\infty \frac{1}{2^n}$$ diverges because you can write it as
$$\sum_{n=1}^\infty \left(\frac{1}{2}\times\frac{1}{2^{n-1}}\right)$$ and $$\sum_{n=1}^\infty \frac{1}{2}$$ diverges.

3. Feb 26, 2012

### DarkStalker

Point. I just checked my textbook and it appears I'd mistakenly thought that just because Summation 1-infinity (ai+bi) equals summation 1-infinity (ai) + Summation 1-infinity (bi), I thought the same would be true for multiplication.

Thanks for clearing it up.

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