11. $\sum_{k=1}^{2 n-1}(-1)^{k-1} \frac{n-k}{C_{2 n}^{k}}=\sum_{k=1}^{n-1}(-1)^{k-1} \frac{n-k}{C_{2 n}^{k}}-\sum_{i=1}^{n-1}(-1)^{i-1} \frac{n-i}{C_{2 n}^{i}}=0$.
11. Calculate: $\sum_{k=1}^{2 n-1}(-1)^{k-1} \cdot \frac{n-k}{C_{2 n}^{k}}$.