ArcGIS API for Flex asks the start, end, and midpoint of a line segment

To find the midpoint of a line segment:
Get Polyline's middle Path first, and then get the middle point of the path as the midpoint of the entire polyline.

Private Function Getcenter (pl:polyline): mappoint{
    var pathcount:number=pl.paths.length;
    var pathindex:int=int ((PATHCOUNT/2)-1);
    var midpath:array=pl.paths[pathindex];
    var ptcount:number=midpath.length;
    var ptindex:int=int ((PTCOUNT/2)-1);
    var pt:mappoint=pl.getpoint (Pathindex, ptindex);
    return pt;
}

To find the beginning and end of a line segment:

var Xx:int=luduanlist.selectedindex;
				var mygraphic:graphic=graphicarr[xx];
				var l:polyline=mygraphic.geometry as Polyline;
				
				
				var goldpath:array=l.paths[l.paths.length-1];
				
				var spt:mappoint=l.getpoint (0,0);
				
				
				
				
				
				var mpt:mappoint=l.getpoint (l.paths.length-1,goldpath.length-1);