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

theorem
  b < 0 & d < 0 & 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,Th80;
  then a*d*b<c;
  hence thesis by A1,Th82;
end;
