p=sinθ
p^2=psinθ
x^2+y^2=y
x^2+(y-1/2)^2=1/4.圆心:(0,1/2).
p=cosθ
p^2=pcosθ
x^2+y^2=x
(x-1/2)^2+y^2=1/4,圆心：（1/2,0）.
圆心距=√2/2.