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