What are the lengths of the diagonals of a rhombus whose each side is $1 \mathrm{~m}$ and one of its angles is $120^{\circ}$?
The diagonal divides the rhombus into two equilateral triangles, because the acute angle of the rhombus is $180^{\circ}-120^{\circ}=60^{\circ}$, and thus one of the diagonals is $1 \mathrm{~m}$.

The double height of the equilateral triangle gives the size of the other diagonal, so its magnitude is

$$
2 \cdot \frac{1}{2} \sqrt{3}=\sqrt{3}=1.732 \mathrm{~m}
$$

(Milhofer Lajos, Pécs.)