theorem Satz8p9:
  right_angle a,b,c & Collinear a,b,c implies a = b or c = b
  proof
    assume
A1: right_angle a,b,c & Collinear a,b,c;
    assume
A2: a <> b;
    Collinear b,a,c by A1,GTARSKI3:45;
    hence thesis by A1,A2,Satz8p3,Satz8p8;
  end;
