5. 为缺项级数，记收敛半径为R，则R^2 = lima/a = lim(2n)![(n+1)!]^2 / {[2(n+1)]!(n!)^2}= lim(n+1)^2 / [(2n+2)(2n+1)]= lim(n+1) / [2(2n+1)] = 1/4,收敛半径 R = 1/2x = ±1/2 时，级数发散。收敛域是 (-1/2, 1/2)
