分母是:1+3*N分子是:(2^N+1)*(-1)^(N+1)
-(2^N+1)*(-1)^(N)————————— 1+3*N哈哈哈同 chenlu8178上解
当N为基数时为 (1+2的n次方)/(1+3n)当N为偶数时为 -(1+2的n次方)/(1+3n)