lim{(1/1-x)-(3/1-x³)}= lim{(1/1-x)-(3/(1-x)(1+x+x²)}= lim{(1+x+x²-3)/[(1-x)(1+x+x²)]}= lim{[(x-1)(x+2)]/[(1-x)(1+x+x²)]}= lim[(x+2)/(1+x+x²)]=1(x→1)
