r和那个角度θ的取值范围是根据图形来取值的。例如，对圆x^2+y^2=4,0<=r<=2,0<=θ<=2π；对圆(x-1)^2+y^2=1,-π/2<=θ<=π/2,0<=r<=2cosθ.
r²=x²+y²
