解:(1)当a=-1时,B={xx2-2x-3>0}={xx<-1或x>3}因为A={x1≤x≤5},所以A∪B1或x≥1}分2)由x2-(a+3)x+3a>0,得(x-a)(x-3)>0当a=3时,B={x|x≠3,x∈R},满足A∪B=R.当a>3时,B={xx<3或x>a},因为AUB=R,所以3