Aus Online Mathematik Brückenkurs 1

If we write the equation as

\displaystyle (x-0)^2 + (y-0)^2 = 9

we can interpret the left-hand side as the square of the distance between the points (x,y) and (0,0). The whole equation says that the distance from a point (x,y) to the origin should be constant and equal to \displaystyle \sqrt{9}=3\,, which describes a circle with its centre at the origin and radius 3.