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

theorem
  0 < b & 0 < d & a*b < c/d implies a*d < c/b
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,Th81;
end;
