∑ i = 0 n a i ∑ j = 0 i ( n k ) x n − j ( − 1 ) j {\displaystyle \sum _{i=0}^{n}a_{i}\sum _{j=0}^{i}{n \choose k}x^{n-j}(-1)^{j}}
∑ i = 0 n ( n k ) x n − i ( − 1 ) i ∑ j = i n a j {\displaystyle \sum _{i=0}^{n}{n \choose k}x^{n-i}(-1)^{i}\sum _{j=i}^{n}a_{j}}