Problem 9. The polyhedron shown in the figure has all dihedral angles that are right angles. Sasha claims that the shortest path along the surface of this polyhedron from vertex $X$ to vertex $Y$ has a length of 4. Is he correct?

![](https://cdn.mathpix.com/cropped/2024_05_06_c3e6e773b77df3772054g-10.jpg?height=380&width=334&top_left_y=104&top_left_x=1569)
Answer: No.

Solution. Consider the development of the three right faces of the figure and connect $X$ and $Y$ with a straight line. The resulting path has a length of $\sqrt{13}$, which is less than 4.