reserve th, th1, th2, th3 for Real;

theorem Th6:
  cos(th)<>0 implies sin(th) = cos(th)*tan(th)
proof
  assume cos(th)<>0;
  then sin(th)= (cos(th)/cos(th))*sin(th) by XCMPLX_1:88
    .= cos(th)*tan(th) by XCMPLX_1:75;
  hence thesis;
end;
