∑i=0x∑j=0y(i+ji)=(x+2)(x+y+2x+2)−y−1y+1\sum_{i=0}^{x} \sum_{j=0}^{y} \binom{i+j}{i} = \frac{(x+2)\binom{x+y+2}{x+2}-y-1}{y+1} ∑i=0x∑j=0y(ii+j)=y+1(x+2)(x+2x+y+2)−y−1
