1. Given that the lengths of the two diagonals of a trapezoid are $m$ and $n$, and the angle between the two diagonals is $60^{\circ}$. Then the area of the trapezoid is ( ).
(A) $\frac{\sqrt{3}}{8} m n$
(B) $\frac{\sqrt{3}}{4} m n$
(C) $\frac{\sqrt{3}}{2} m n$
(D) $\sqrt{3} m n$
1. B As shown in the figure, let $A C=m, B D=n$. Extend $A B$ to $E$ such that $B E=C D$. Then we have
$$
C E=B I=n \cdot \angle A C E=
$$
$\angle A O B$ is $60^{\circ}$ or $120^{\circ}$,
Also, $S_{\triangle A C D}=S_{\triangle H C E}$, thus
$$
S_{\triangle M E E}=\frac{1}{2} A C \cdot C E \sin \angle A C E=\frac{\sqrt{3}}{4} m n .
$$