Use the ratio test to find the radius of convergence for ∑n=1∞n2nxn\sum_{n=1}^{\infty} \frac{n}{2^n}x^n∑n=1∞2nnxn.
R=1R = 1R=1
R=2R = 2R=2
R=12R = \frac{1}{2}R=21
R=∞R = \inftyR=∞