The radius is something you only find in circles or spheres .When you start at the center point and go to a point on the surface,the line segment you made is always the same length, the radius.
The apothem is used in polygons. This is because when you draw a line segment between the center of a polygon and a point on the edge, it can take on different lengths. the apothem though is always the one thats perpendicular to a side.
