1979 IMO 第 4 题

(BUL 3) ${ }^{\mathrm{IMO} 2} \mathrm{~A}$ pentagonal prism $A_{1} A_{2} \ldots A_{5} B_{1} B_{2} \ldots B_{5}$ is given. The edges, the diagonals of the lateral walls and the internal diagonals of the prism are each colored either red or green in such a way that no triangle whose vertices are vertices of the prism has its three edges of the same color. Prove that all edges of the bases are of the same color.

(BUL 3) ${ }^{\mathrm{IMO} 2} \mathrm{~A}$ 五角棱镜 $A_{1} A_{2} \ldots A_{5} B_{1} B_{2} \ldots B_{5}$ 已给出。棱柱的边、侧壁的对角线和内部对角线均被着色为红色或绿色，使得顶点为棱柱的顶点的三角形的三个边的颜色相同。证明底边的所有边都是相同的颜色。