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

theorem
  b < 0 & 0 < d & a*b < c/d 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,Th79;
  then a*d*b<c;
  hence thesis by A1,Th82;
end;
