at (a) AB: SD()->r+DD BC: SD(a)->da+aD CA:D(B)B→B+BD at (d) AD: D+DB=yB=BC 3 BD: aD+ Dy=ay=CA 3 CD: BD+Da=Ba=AB 3CA D D BC D D AB D D a                   ⎯→ + ⎯→ + ⎯→ + : ( ) : ( ) : ( ) at ( ) CD D D AB BD D D CA AD D D BC d 3 1 : 3 1 : 3 1 : at ( ) + = = + = = + = =            