code: def _mass_from_knownmass_eta(known_mass, eta, known_is_secondary=False,
                              force_real=True):
      r
      roots = numpy.roots([eta, (2*eta - 1)*known_mass, eta*known_mass**2.])
      if force_real:
          roots = numpy.real(roots)
      if known_is_secondary:
          return roots[roots.argmax()]
      else:
          return roots[roots.argmin()]
docs: r"""Returns the other component mass given one of the component masses
      and the symmetric mass ratio.
  
      This requires finding the roots of the quadratic equation:
  
      .. math::
          \eta m_2^2 + (2\eta - 1)m_1 m_2 + \eta m_1^2 = 0.
  
      This has two solutions which correspond to :math:`m_1` being the heavier
      mass or it being the lighter mass. By default, `known_mass` is assumed to
      be the heavier (primary) mass, and the smaller solution is returned. Use
      the `other_is_secondary` to invert.
  
      Parameters
      ----------
      known_mass : float
          The known component mass.
      eta : float
          The symmetric mass ratio.
      known_is_secondary : {False, bool}
          Whether the known component mass is the primary or the secondary. If
          True, `known_mass` is assumed to be the secondary (lighter) mass and
          the larger solution is returned. Otherwise, the smaller solution is
          returned. Default is False.
      force_real : {True, bool}
          Force the returned mass to be real.
  
      Returns
      -------
      float
          The other component mass.