|x-1|=根号(1-(y-1)^2)
(x-1)^2=1-(y-1)^2 1-(y-1)^2≥0
(x-1)^2+(y-1)^2=1 (y-1)^2≤1
(x-1)^2+(y-1)^2=1 0≤y≤2
表示以(1,1)为圆心,1为半径的圆