Half a Circle. The Perimeter of the semicircle of Radius is

(1) 
and the Area is

(2) 
The weighted mean of is

(3) 
The Centroid is then given by

(4) 
The semicircle is the CrossSection of a Hemisphere for any Plane through the zAxis.
See also Arbelos, Arc, Circle, Disk, Hemisphere, Lens, Right Angle,
Salinon, Thales' Theorem, YinYang
© 19969 Eric W. Weisstein
19990526