A regular pentagon and a right triangle. What’s the angle α?
It’s all right II

A regular pentagon and a right triangle. What’s the angle α?
Two intersecting circles with two inscribed triangles and two line segments through an intersection point. Prove that the triangles are similar.
Co-author: Marshall W. Buck.
An equilateral triangle with two perpendiculars to a point that is at the same height as the triangle centre. What is red : yellow?
Behold a 13-14-15 triangle. The 15-long side is dissected into 5 equal parts and the 14-long side is dissected into 7 equal parts. What is the difference between the red and green areas?
Author: Marshall W. Buck.
Two rectangles share two vertices. What’s the green fraction?
A triangle, its incircle and a right triangle. The circle centre is shown in blue. Prove that the tangency points and right triangle vertex are collinear.
Two blue squares and two red right triangles. What are the proportions of the triangle sides?
Author: Matthew Arcus.
Two squares share a vertex. What’s blue : green : orange?
An equilateral triangle and three line segments. What’s the angle α?
Two prisoners are chained to the walls of a triangular cell. The base is covered with a mirror and a perpendicular wall is separating them. They can each see the opposite corner through a hole. Proof they can see each other through another hole at the base of the perpendicular.
Proposed by Marshall W. Buck. Note that this problem is known as the Blanchet Theorem.