(3) R = lima/a = lim5^n(n+1)^2/[5^(n+1)n^2] = 1/5, x = ±1/5 时级数为 ∑1/x^2 收敛，则收敛域 [-1/5, 1/5].(4) R = lim|a/a| = lim2^(n+1)(n+2)/[2^n(n+1)] = 2, x = 2 时级数为 ∑(-1)^n/(n+1) 收敛，x = -2 时级数为 ∑1/(n+1) 发散。则收敛域 (-2，2].
