例: ∑ k=1 n 1 k( k+1 )
∑ k=1 n 1 k( k+1 )( k+2 )
∑ k=1 n n−1 n!
1 k( k+1 )( k+2 ) = 1 2 { 1 k( k+1 ) − 1 ( k+1 )( k+2 ) }
n−1 n! = 1 ( n−1 )! − 1 n!