(x-y)的2n+3次方÷(y-x)的2n次方÷(y-x)的3次方
=-(y-x)的2n+3次方÷(y-x)的2n次方÷(y-x)的3次方
=-(y-x)的[(2n+3)-2n-3]次方
=-(y-x)的0次方
=-1