令t=根号(1-x)(t≥0)
则y=-t^2-2t+3=-(t+1)^2+4
∵t≥0
∴t+1≥1
∴(t-1)^2≥1
∴-(t+1)^2≤-1
∴y=-(t+1)^2+4≤3
值域为(-∞,3]