reserve a,b,c,d for Real;
reserve r,s for Real;

theorem
  0 < b & 0 < d & c/d < a*b implies c/b < a*d
proof
  assume that
A1: b>0 and
A2: d>0;
  assume a*b>c/d;
  then a*b*d>c by A2,Th77;
  then a*d*b>c;
  hence thesis by A1,Th83;
end;
