lim0→∞ [(2n+1)^20 •(3n-1)^30] /(5n-1)^50 (分子分母同除以n^50)
=lim0→∞ [(2+1/n)^20 •(3-1/n)^30] /(5-1/n)^50
=2^20×3^30/5^50