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

theorem
  0 < b & d < 0 & a*d < c*b implies c/d < a/b
proof
  assume that
A1: b>0 and
A2: d<0 and
A3: a*d<c*b;
  a*d/b<c by A1,A3,Th83;
  then a/b*d<c by XCMPLX_1:74;
  hence thesis by A2,Th82;
end;
