2019 IMO Shortlist G5

Let $A B C D E$ be a convex pentagon with $C D=D E$ and $\angle E D C \neq 2 \cdot \angle A D B$. Suppose that a point $P$ is located in the interior of the pentagon such that $A P=A E$ and $B P=B C$. Prove that $P$ lies on the diagonal $C E$ if and only if $\operatorname{area}(B C D)+\operatorname{area}(A D E)=$ $\operatorname{area}(A B D)+\operatorname{area}(A B P)$. (Hungary)

令 $A B C D E$ 为凸五边形，其中 $C D=D E$ 且 $\angle E D C \neq 2 \cdot \angle A D B$。假设点 $P$ 位于五边形的内部，使得 $A P=A E$ 且 $B P=B C$。证明$P$位于对角线$C E$上当且仅当$\operatorname{area}(B C D)+\operatorname{area}(A D E)=$ $\operatorname{area}(A B D)+\operatorname{area}(A B P)$。 (匈牙利)