Find the sum of the series ∑n=1∞∑k=1∞(12)n(13)k\sum_{n=1}^{\infty} \sum_{k=1}^{\infty} (\frac{1}{2})^n (\frac{1}{3})^k∑n=1∞∑k=1∞(21)n(31)k.
1/21/21/2
1/41/41/4
111
2/32/32/3