We need the following Lemma 1. For every different points $A, B, C, D$ the inequality $A B \cdot C D+B C \cdot A D \geqslant A C \cdot B D$ holds. Proof. Consider the point $A_{1}$ on the ray $D A$ such that $D A_{1}=\frac{1}{D A}$. In the same way we take the points $B_{1}$ and $C_{1}$ on the rays $D B$ and $D C$. Since $\frac{D A_{1}}{D B}=\frac{D B_{1}}{D A}=\frac{1}{D A \cdot D B}$, it follows from similarity of the triangles $D A B$ and $D B_{1} A_{1}$ that $A_{1} B_{1}=\frac{A B}{D A \cdot D B}$. Similarly, $B_{1} C_{1}=\frac{B C}{D B \cdot D C}$ and $C_{1} A_{1}=\frac{C A}{D C \cdot D A}$ (1). Substituting these equalities in the triangle inequality $A_{1} B_{1}+B_{1} C_{1} \geqslant A_{1} C_{1}$ we obtain $A B \cdot C D+B C \cdot A D \geqslant A C \cdot B D$. Lemma 2. For every points $M, N$ in the plane of the triangle $A B C$ $$\frac{A M \cdot A N}{A B \cdot A C}+\frac{B M \cdot B N}{B A \cdot B C}+\frac{C M \cdot C N}{C A \cdot C B} \geqslant 1$$ Proof. In the plane $A B C$ we consider the point $K$ such that $\angle A B M=\angle K B C, \angle M A B=\angle C K B$. Note that $$\frac{C K}{B K}=\frac{A M}{A B}, \frac{A K}{B K}=\frac{C M}{B C}, \frac{B C}{B K}=\frac{B M}{A B}$$ Applying lemma 1 to the points $A, N, C, K$ we have $A N \cdot C K+C N \cdot A K \geqslant A C \cdot N K$. Triangle inequality $N K \geqslant B K-B N$ gives us $A N \cdot C K+C N \cdot A K \geqslant A C \cdot(B K-B N)$. Hence we obtain $$\frac{A N \cdot C K}{A C \cdot B K}+\frac{C N \cdot A K}{A C \cdot B K}+\frac{B N}{B K} \geqslant 1$$ It follows from (3) and (2) that $\frac{A M \cdot A N}{A B \cdot A C}+\frac{B M \cdot B N}{B A \cdot B C}+\frac{C M \cdot C N}{C A \cdot C B} \geqslant 1$. Corollary. The inequality remains true when one of the points $M, N$, or both, lie outside the plane of the triangle $A B C$. It follows from lemma 2 when instead of $M$ and $N$ it is applied to their projections onto the plane of the triangle $A B C$. We are ready now to solve the problem. On the ray $D A$ we consider the point $A_{1}$ such that $D A_{1}=\frac{1}{D A}$. In a similar way we take points $B_{1}, C_{1}, M_{1}, N_{1}$ on the rays $D B, D C, D M, D N$. Applying the corollary of Lemma 2 to the points $M_{1}, N_{1}$ and the triangle $A_{1} B_{1} C_{1}$ we get the inequality $A_{1} M_{1} \cdot A_{1} N_{1}+B_{1} M_{1} \cdot B_{1} N_{1}+C_{1} M_{1} \cdot C_{1} N_{1} \geqslant A_{1} B_{1}^{2}$; using equations similar to (1) we obtain $$\frac{A M}{D A \cdot D M} \cdot \frac{A N}{D A \cdot D N}+\frac{B M}{D B \cdot D M} \cdot \frac{B N}{D B \cdot D N}+\frac{C M}{D C \cdot D M} \cdot \frac{C N}{D C \cdot D N} \geqslant\left(\frac{A B}{D A \cdot D B}\right)^{2}$$ whence $$A M \cdot A N+B M \cdot B N+C M \cdot C N \geqslant D M \cdot D N$$ q.e.d.