Polygons are closed shapes made from straight line segments, and they appear throughout geometry, design, engineering, and computer graphics. One of the most important ways to classify a polygon is by deciding whether it is convex or concave. A convex polygon has no inward dents, while a concave polygon has at least one inward dent.
This difference affects angles, diagonals, area methods, and how shapes behave in visual and mathematical tests.
A convex polygon passes the angle test because every interior angle is less than 180 degrees, and it passes the diagonal test because every diagonal stays inside the polygon. A concave polygon fails these tests because at least one interior angle is greater than 180 degrees, called a reflex angle, and at least one diagonal lies partly outside the polygon. These tests give students a reliable way to identify the shape without relying only on appearance.
Understanding convex and concave polygons also helps with triangulation, coordinate geometry, and recognizing valid shapes in real-world diagrams.
Key Facts
- A polygon is convex if every interior angle is less than 180 degrees.
- A polygon is concave, or non-convex, if at least one interior angle is greater than 180 degrees.
- Interior angle sum of an n-sided polygon: S = (n - 2)180 degrees.
- Number of diagonals in an n-sided polygon: D = n(n - 3)/2.
- Convex diagonal test: every diagonal lies completely inside the polygon.
- Concave diagonal test: at least one diagonal lies partly outside the polygon.
Vocabulary
- Polygon
- A polygon is a closed two-dimensional figure made from three or more straight line segments.
- Convex Polygon
- A convex polygon is a polygon in which every interior angle is less than 180 degrees and every diagonal stays inside the shape.
- Concave Polygon
- A concave polygon is a polygon with at least one interior angle greater than 180 degrees, creating an inward dent.
- Reflex Angle
- A reflex angle is an angle greater than 180 degrees and less than 360 degrees.
- Diagonal
- A diagonal is a line segment that connects two non-adjacent vertices of a polygon.
Common Mistakes to Avoid
- Calling any irregular polygon concave. This is wrong because an irregular polygon can still be convex if all interior angles are less than 180 degrees.
- Checking only one angle in a polygon and assuming the classification. This is wrong because a polygon is concave if any one interior angle is greater than 180 degrees, so every angle matters.
- Using curved sides when identifying polygons. This is wrong because polygons must be made only of straight line segments.
- Counting sides instead of testing angles or diagonals. This is wrong because triangles, quadrilaterals, pentagons, and larger polygons can be classified by convexity only through their angles or diagonals.
Practice Questions
- 1 A pentagon has interior angles 100 degrees, 110 degrees, 95 degrees, 230 degrees, and 185 degrees. Is it convex or concave, and why?
- 2 How many diagonals does an octagon have? Use D = n(n - 3)/2, then explain whether this formula depends on the polygon being convex.
- 3 A polygon has one vertex that bends inward, but all of its sides are straight and the shape is closed. Explain which visual tests could prove that it is concave.