HG 33
T51. The angle sum of every quadrilateral is less than 360 degrees.
T52. No rectangles exist.
D 22. In triangle ABC, let D be the midpoint of AB, E be the midpoint of AC, and F, G, H, the projections of B, A, C on line DE. Quadrialteral BFHC is called an associated Saccheri quadrilateral (ASQ).
T53. The angle sum of a triangle equals the sum of the summit angles in each ASQ.