X2+Y2=2Y
x^2+(y-1)^2=1圆心（0,1）半径R=1
参数方程：
x=cost,y=1+sint
x+y+a
=cost+sint+1+a
=√2sin(t+45°）+1+a>=0
-√2+1+a>=0
a>=√2-1