theorem Th15:
  tan r = (cot r)" & cot r = (tan r)"
  proof
    thus tan r = (sin r*(cos r)")" "
    .= ((sin r)"*(cos r)" ")" by XCMPLX_1:204
    .= (cot r)";
    thus cot r = (cos r*(sin r)")" "
    .= ((cos r)"*(sin r)" ")" by XCMPLX_1:204
    .= (tan r)";
  end;
