原式=lim(x→∞)(1+x+x²)/(1-x)(1+x+x²)-1//(1-x)(1+x+x²)]
=lim(x→∞)(x+x²)/(1-x³)
上下除以x³
=lim(x→∞)(1/x²+1/x)/(1/x³-1)
=（0+0）/（0-1）
=0
limx趋于正无穷 [1/(1-x)-1/(1-x^3)] 求函数极限
原式=lim(x→∞)(1+x+x²)/(1-x)(1+x+x²)-1//(1-x)(1+x+x²)]
=lim(x→∞)(x+x²)/(1-x³)
上下除以x³
=lim(x→∞)(1/x²+1/x)/(1/x³-1)
=（0+0）/（0-1）
=0