Using blahtexml --xmlin

In general, the distance of a point with coordinates $(x,y)$ to the origin $(0,0)$ is $\sqrt{x^2+y^2}$. A circle of radius $r$ is the set of points that are at a distance $r$ of its center. So, the equation of a circle of radius $r$ centered at the origin is $$x^2+y^2=r^2\text{.}$$