1.
(x-3)^2/(3-x)*(x+3)^2/(x+3)
=(3-x)*(x+3)
=9-x^2
2.化简求值：
[(a^2-1)/(a^2-a-2)]/[a/(2a-4)] (其中a=1/2)
原式=[(a+1)*(a-1)/(a+1)*(a-2)]/[a/2(a-2)]
=[(a-1)/(a-2)]/[a/2(a-2)]
=(a-1)*2(a-2)/(a-2)*a
=(a-1)/a
=1-(1/a)
3.若x=2005,y=2006.则
(x+y)*(x^2+y^2)/(x^4-y^4)=?
原式=(x+y)*(x^2+y^2)/(x^2+y^2)*(x^2-y^2)
=(x+y)/(x^2-y^2)
=(x+y)/(x+y)*(x-y)
=1/(x-y)
=1/(2005-2006)
=-1