先进行简化,再代入即可：
原式=（x+y)*(x*x+y*y)/(x*x*x*x-y*y*y*y)
=（x+y)*(x*x+y*y)/((x*x+y*y)(x*x-y*y))
=(x+y)/(x*x-y*y)
=(x+y)/((x+y)*(x-y))
=1/(x-y)
=1/(2007-2008)
=-1