圆心直角坐标(rcos(3π/2),rsin(3π/2))=(0,-r)
圆的直角坐标方程：(x+0)^2+(y+r)^2=r^2
化成极坐标方程：(pcosa)^2+(psina+r)^2=r^2
p^2+2rpsina=0
p+2rsina=0
p=-2rsina