Polar radius
= Polar radius
{title2=$r=\sqrt{x^2+y^2}$}
In planar <polar coordinates> $(r,\theta)$, the polar radius $r\geq0$ is the <Euclidean distance> to the origin. It satisfies $x=r\cos\theta$ and $y=r\sin\theta$. For a differentiable <plane curve> with parameters $r(t),\theta(t)$, the orthogonality of radial and angular directions gives squared <speed> $\dot r^2+r^2\dot\theta^2$.