def _mass_from_knownmass_eta(known_mass, eta, known_is_secondary=False,
                            force_real=True):
    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.
    """
    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()]