(10) ρ = lima/a = lim4^(n+1)(5^n-3^n)/{[5^(n+1)-3^(n+1)]4^n}, 分子分母同出于(4^n 5^n) ρ = lim 4[1-(3/5)^n] / [5 - 3(3/5)^n] = 4/5 < 1 级数收敛。
