Polygons
Shapes with straight sides come with three, four, five or more corners. What do they share, and what changes as the number of sides grows?
Where you are 3 inside
What this area is about
A polygon is a flat, closed figure made of straight sides that meet only at their ends. Counting the sides names it, and the same count fixes how much its interior angles add up to. That angle total in turn helps decide which polygons can cover a floor with no gaps.
How these fit together
Names. Naming and classifying polygons says what a polygon is, names it by its number of sides (pentagon, hexagon, octagon), and sorts polygons into convex and concave, regular and not.
A result. Angle sums of polygons cuts a polygon into triangles to show that the interior angles of a polygon with n sides add to (n - 2) \cdot 180^\circ, and also counts diagonals and exterior angles.
A use. Tessellations asks which shapes tile the plane: where tiles meet corner to corner at a point, their angles must add to exactly 360^\circ.
A path through it
Angle sums come before tessellations, since the tiling question is settled by the angles at each corner. Naming polygons has no fixed place in that order.
Finish here and you are ready for regular polygons and their angles, cyclic quadrilaterals and polyhedra.