已知∑i=0xCxi∗Cyz+i\begin{aligned}\sum_{i=0}^xC_{x}^i*C_{y}^{z+i}\end{aligned}i=0∑xCxi∗Cyz+i,求证化简完为Cx+yx+z\begin{aligned}C_{x+y}^{x+z}\end{aligned}Cx+yx+z,考试的时候打表打出来的规律但是整证不出来(有无组合数dalao帮忙证一下