(1)a n=3, it is a triangle, and the sum of its internal angles is 180.
Suppose there is more than one right angle or obtuse angle, and the sum of its internal angles is >: 90+90 = 180, which is contradictory!
So there is only one right angle or obtuse angle at most.
When b n=4, it is quadrilateral, and the sum of internal angles is 360. Obviously, four right angles can be satisfied, such as a rectangle.
Suppose there are more than three obtuse angles, and the sum of their internal angles >; 90 ? 4 = 360, contradiction!
So there are only three obtuse angles at most.
When c n≥5, there are at most three right angles or obtuse angles, which is incorrect. Such as regular pentagons, regular hexagons, etc. Their internal angles are obtuse.
(2) If there are more than three acute angles, the sum of their internal angles is
So the assumption is not true, that is, the acute angle cannot exceed three.