1991 IMO 第 4 题

(FRA 2) ${ }^{\mathrm{IMO5}}$ Let $A B C$ be a triangle and $M$ an interior point in $A B C$. Show that at least one of the angles $\measuredangle M A B, \measuredangle M B C$, and $\measuredangle M C A$ is less than or equal to $30^{\circ}$.

(FRA 2) ${ }^{\mathrm{IMO5}}$ 设$A B C$ 为三角形，$M$ 为$A B C$ 中的内点。证明角度 $\measuredangle M A B、\measuredangle M B C$ 和 $\measuredangle M C A$ 中的至少一个小于或等于 $30^{\circ}$。