Proof: If the fourth angle were obtuse, our quadrilateral would have an angle sum greater
than 360◦
, which cannot happen. If the angle were a right angle, then a rectangle would
exist and all triangles would have to have defect 0. Since there is a triangle with angle sum
less than 180◦
, we have a triangle with positive defect. Thus, the fourth angle cannot be a
right angle either.