12.23\frac{2}{3}32,415\frac{4}{15}154,635\frac{6}{35}356,863\frac{8}{63}638,1099\frac{10}{99}9910,…\ldots…的一个通项公式是 ___2n(2n−1)(2n+1)\frac{2n}{(2n-1)(2n+1)}(2n−1)(2n+1)2n___.