Question 5
In a tetrahedron, the product of the lengths of each pair of opposite edges is $l$. Let $\alpha,\beta,\gamma$ be the angles between the three pairs of opposite edges, and let $R_1,R_2,R_3,R_4$ be the circumradii of its four faces. Prove
$$\sin^2\alpha+\sin^2\beta+\sin^2\gamma\ge\frac{l}{\sqrt{R_1R_2R_3R_4}}.$$
$$\sin^2\alpha+\sin^2\beta+\sin^2\gamma\ge\frac{l}{\sqrt{R_1R_2R_3R_4}}.$$