Chapter 8: Quadrilaterals

Quadrilaterals applies the triangle congruence criteria just learned to a new shape family — parallelograms, rectangles, rhombuses and squares — with most proofs working by drawing a diagonal to split the quadrilateral into two triangles first.

Assuming a property true for one type of quadrilateral (like a rhombus's perpendicular diagonals) automatically applies to every parallelogram is a frequent overreach — it doesn't, unless stated; an ordinary parallelogram's diagonals bisect each other but aren't perpendicular. The mid-point theorem introduced here, connecting a triangle's side midpoints, reappears later in coordinate geometry and area problems.