设：
lim(n→∞) A(n+1)/An =ρ1 ==> R1= 1/ρ1=(5^(1/2))/3
lim(n→∞) B(n+1)/Bn =ρ2 ==> R2= 1/ρ2=1/3
则：
lim(n→∞) [(B(n+1)^2/A(n+1)^2)]/(Bn^2/An^2)
=lim(n→∞) [(B(n+1)^2/Bn^2)]/(A(n+1)^2/An^2)
=ρ2^2/ρ1^2
=R1^2/R2^2
=((5^(1/2))/3)^2/(1/3)^2
=ρ
∑（n=1 → ∞） (Bn^2/An^2) x^n的收敛半径
R= 1/ρ =1/5
选 D