Three concurrent lines with direction cosines $l_1,m_1,n_1; l_2,m_2,n_2$ and $l_3,m_3,n_3$ are coplanar, then $\left|\begin{array}{lll}{l}_1& m_1& n_1\\ l_2& m_2& n_2\\ l_3& m_3& n_3\end{array}\right|=$

Two systems of rectangular axes have the same origin. If a plane cuts them at the distance $a,b,c$ and $a^{\text{'}},b^{\text{'}},c^{\text{'}}$ respectively from the origin, then $\frac{1}{a^2}+\frac{1}{ b^2}+\frac{1}{c^2}=\mathrm{k}\left(\frac{1}{a^{\text{'}2}}+\frac{1}{\mathit{ }{b}^{\text{'}2}}+\frac{1}{c^{\text{'}2}}\right)$, where $k=$