该加的括号加上啊.是不是y=√(-x^2+2x+3)?
定义域：-x^2+2x+3
=-(x^2-2x-3)
=-(x^2-2x+1-4)
=-(x^2-2x+1)+4
=-(x-1)^2+4
可得-(x-1)^2+4>=0,(x-1)^2