limx->∞原式
=limx->∞[x^30(2-1/x)^30*x^20(3+2/x)^20]/x^50(5+1/x)^50
=limx->∞(2-1/x)^30*(3+2/x)^20/(5+1/x)^50
=[(2-0)^30*(3+0)^20]/(5+0)^50
=2^30*3^20/5^50