15-2x-x^2>； =0
x^2+2x- 15<； =0
(x+5)(x-3)& lt； =0
-5 = & lt； x & lt=3
Domain: A=
CrA=(-∞，-5)U(3，+∞)
y=a-2x-x^2
=-(x+ 1)^2+a+ 1
The opening is downward, and the maximum value is ymax=a+ 1.
Range: B=(-∞, a+ 1]
CrAUB = R
∴a+ 1>； =3
a & gt=2
The value range of a: [2, +∞)