\displaystyle \sum_{k = 2 n + 1}^{\infty} \frac{1}{2^k} = \sum_{k = 0}^{\infty} \frac{1}{2^{k + 2 n + 1}} = \frac{1}{2^{2 n + 1}} \sum_{k = 0}^{\infty} \frac{1}{2^k} = \frac{1}{2^{2 n + 1}} \cdot 2 = \frac{1}{2^{2 n}}