x^2+y^2-2x-4y-4=0
(x-1)^2+(y-2)^2=9
令x=1+3cosa,y=2+3sina
x+y=3(1+cosa+sina)
=3+3(sina+cosa)
=3+3√2sin(a+π/4)
因为-1≤sin（a+π/4）≤1
所以3-3√2≤x+y≤3+3√2