直角坐标方程为 x^2+(y-a)^2=a^2 => x^2+y^2-2ay+a^2=a^2 => x^2+y^2=2ay
∵ y=ρsinθ x^2+y^2=ρ^2
∴ ρ^2=2aρsinθ => ρ=2asinθ