∑i=02n(−1)i(C2ni)2=(−1)n+1C2nn\sum_{i=0}^{2n} (-1)^i (C_{2n}^i)^2 = (-1)^{n+1} C_{2n}^n∑i=02n(−1)i(C2ni)2=(−1)n+1C2nn