-x^2-2x+3≥0→-3≤x≤1 ,即[-3,1]是它是定义域;
f(x)=根号下-x^2-2x+3 =√[-(x+1)^2 +4] ≤√4 =2;
而f(-3)=f(1)=0;
故其值域为[0,2]