求证 12n∑i=0nC(n,i)i2=(n+1)×n4\frac{1}{2^n} \sum\limits_{i=0}^{n} C(n,i)i^2=\frac{(n+1)\times n}{4}2n1i=0∑nC(n,i)i2=4(n+1)×n https://www.luogu.com.cn/discuss/922428
