lim(x→∞)[(ax³+13x²)/(bx⁴+3x³+8)=1b=0 (否则，极限=0）lim(x→∞)[(ax³+13x²)/(bx⁴+3x³+8)=1=lim(x→∞)[(ax³+13x²)/(3x³+8)=1a=3
