n/2 points of [color=#0000ff]A[sub]1[/sub],...,A[sub]n[/sub][/color] may be moving on the surface of a sphere of radius r. Each point has [color=#ff7700]antipodal point[/color] on a sphere . Thus, the [color=#ff00ff]center of gravity[/color] is the centre of the sphere.[br]  It is shown, that the sum of squares of distances from any point D of a sphere to →{[color=#0000ff]A[sub]i[/sub][/color]} , divided by radius squared, is always equal of twice the number of points.[br][url=https://en.wikipedia.org/wiki/Parallel_axis_theorem]https://en.wikipedia.org/wiki/Parallel_axis_theorem[/url][br][url=https://en.wikipedia.org/wiki/Antipodal_point]https://en.wikipedia.org/wiki/Antipodal_point[/url]