[2x／（x³-1）]÷[1／（x²+x+1）]
=(2x)(x²+x+1)/(x³-1)
=(2x)(x²+x+1)/[(x-1)(x²+x+1)]
=2x/(x-1)
=(2x-2+2)/(x-1)
=2+ 2/(x-1)