隔项级数，则收敛半径的平方是R^2 = lim|a/a| = lim|n^2[2^(n+1)+(-3)^(n+1)]/{(n+1)^2[2^n+(-3)^n]}|= limn^2/(n+1)^2 lim|[2^(n+1)+(-3)^(n+1)]/[2^n+(-3)^n]|= 1· lim|[2(2/3)^n+(-3)(-1)^n]/[(2/3)^n+(-1)^n]| = 3R = √3
