really wonderful. there are no (n-1)th equition .

by using -a1（b2+c3b3+...+cnbn）/b1-（b2+c3b3...+cnbn）x as the first factor,the all terms with x are crossed out in the sum of all factors.
then the n-1 equitions are next n-1 factors = 1/n of the sum of all factors which do not contain any terms with x.