
theorem Th33:
  for a,b,c,d being Real st b <> 0 holds
    a * b / c * d / b = a * d / c
  proof
    let a,b,c,d be Real;
    assume
A1: b <> 0;
    a * b / c * d / b = a * ((d / c) * (b / b))
                     .= a * ((d / c) * 1) by A1,XCMPLX_1:60;
    hence thesis;
  end;
