theorem Th3:
  r is non zero & s is non zero & t is non zero implies
  ((-r) / (-s)) * ((-t)/(-r)) * ((-s)/(-t)) = 1
  proof
    assume that
A1: r is non zero and
A2: s is non zero and
A3: t is non zero;
    ((-r) / (-s)) * ((-t)/(-r)) * ((-s)/(-t))
        = (r/s) * ((-t)/(-r)) * ((-s)/(-t)) by XCMPLX_1:191
       .= (r/s) * (t/r) * ((-s)/(-t)) by XCMPLX_1:191
       .= (r/s) * (t/r) * (s/t) by XCMPLX_1:191
       .= r/r * s/s * t/t
       .= 1* s/s * t/t by A1,XCMPLX_1:60
       .= 1 * 1 * t/t by A2,XCMPLX_1:60
       .= 1 * 1 * 1 by A3,XCMPLX_1:60;
    hence thesis;
  end;
