f(x)=(x^3+x^2+x-1)/e^(-x) 可用罗比达法则,既分子分母分别求导 lim(x^3+x^2+x-1)/e^(-x) =(3x^2+2x+1)/-e^(-x) =(6x+2)/e^(-x) =6/-e^(-x) x趋于负无穷时,e^(-x)趋于无穷大 故limf(x)(x→负无穷)=0
