A **quadrilateral** is a polygon (i.e. 2D shape) with four edges (or sides) and four vertices or corners. Examples of quadrilaterals are illustrated below.

In this section we’ll focus on **parallelograms** and **trapezoids**. The videos below will prove the following three things:

- The
*diagonals*of a parallelogram bisect each other. - Joining the
*midpoints*of*adjacent sides*of any quadrilateral forms a*parallelogram*. - The line segment joining the midpoints of the non-parallel sides of a trapezoid is parallel to the parallel sides and has a length equal to the mean of the lengths of the parallel sides.

The next video will show how the length of the **line segment** joining the **midpoints** of the **non-parallel sides** is equal to the **average** lengths of the **parallel sides**.

The final video will show you how to prove that the *diagonals* of a parallelogram, namely a rhombus, bisect each other at 90Â°. Remember that a **diagonal** is a line segment connecting non-adjacent vertices. Let’s take a look: