（4（x－y）的五次方－6（x－y）的四次方＋8（x－y）³）÷2（x－y）²
=2(x-y)²[2(x-y)³-3(x-y)²+4(x-y)]÷2(x-y)²
=2(x-y)³-3(x-y)²+4(x-y)