(x+y)n=(x)n+(X)n-1(y)+(x)n-2(y)n+...+(x)2(y)n-2+(x)(y)n-1+(y)n差不多能看懂吧...Cnr=n!/(r!×(n-r)!)
=∑Cnk(1^k×r^(n-k))[0≤k≤n]Cnr=n!/(r!×(n-r)!)
