答案是：负的二分之一的2007次幂
(1/2)^2007-(1/2)^2006
=(1/2)*(1/2)^2006-(1/2)^2006 ｝
=(1/2)^2006*(1/2 - 1)
=(1/2)^2006*(-1/2)
=-(1/2)^2007