When you say quadrilateral, most people probably first think of a square or a rectangle. The two right-angled figures are seen everywhere.

It is your computer screen, a football field, an A4 sheet of paper, etc.

Common to all quadrilaterals is that the angle sum is \(360^\circ\)

$$ \Large \angle A + \angle B + \angle C + \angle D = 360^\circ $$

Rectangle

A rectangle is a quadrilateral where all angles are 90 degrees. This means that the sides are parallel.

Square

If all four sides are of equal length, it is called a square. Thus a square is also a rectangle, and therefore the same calculation rules apply.

Calculation

For a rectangle it is normally necessary to know both sides in order to calculate perimeter and area. Unless it is a square, then one side is enough, since they are equal.

But if you know the diagonal, you can divide the quadrilateral into two right-angled triangles, which gives other possibilities for calculation.

If you have learned about equations, you can turn the formulas around and calculate the sides, provided you know area or perimeter.