octagon, trapezoid, pentagon...
* * * * *
Not true.
A trapezium (or trapeziod) must have at least one acute angle.
Any regular polygon with 5 or more sides has all obtuse angles. However, an irregular polygon can always have one or more non-obtuse angles (acute, right or even reflex).
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If all of the interior angles of the polygon add up to 180 degrees, then the polygon is a triangle.
A regular polygon is a polygon with congruent sides and interior angles.
The sum of the exterior angles of all polygons equals to 360⁰.Since only the sum of the interior angles of all quadrilaterals equals to 360⁰, the polygon must have 4 sides.
The more sides you add to a figure, the more obtuse angles it has, assuming all of the polygons are perfect (all angle measures =, all side lengths =). An equilateral triangle has 0 (all acute), a square has 0 (all right), a pentagon has 5 (all obtuse), a hexagon has 6, a heptagon has 7, and so on. So really, it's a polygon with an infinite number of sides, which would look like a circle.
Anything you like. Any single interior angle of a polygon is indeterminate - the only constraint is on the sum of all the interior angles.