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Yang Lu and Zhang Jing-Zhong 1984
Let $A_1 A_2 A_3 A_4$ be a planar convex quadrangle with diagonals $A_1 A_3$ and $A_2 A_4$. Is there a quadrangle $B_1 B_2 B_3 B_4$ in Euclidean space such that $A_1 A_3<B_1 B_3, A_2 A_4<B_2 B_4$ but $A_i A_j>B_i B_j$ for other edges?
一个平面凸四边形，不存在比它的四条边更长但两条对角线更短的四边形，如何证明？ |
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