9. Geometry

9.02 Measuring angles

9.03 Parallel lines and transversals

9.04 Triangles

9.05 Quadrilaterals

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9.06 Constructions with a compass (Investigation)

9.07 Constructing triangles (Investigation)

9.08 Constructing quadrilaterals (Investigation)

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Middle Years

9.05 Quadrilaterals

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Lesson

Just like we did with triangles, we can take any four points that do not lie on a line and join them with four segments to form a shape. We call this shape a quadrilateral.

There is an enormous variety of four-sided shapes. The one thing they all have in common is that they can always be split down the middle to make two triangles. This means that:

The angle sum of a quadrilateral is $360^\circ$360°.

The two triangles each form a straight angle, and together they form a full revolution.

Did you know?

Mathematical language can be confusing sometimes. A shape with three sides is called a triangle, because it has three ("tri") angles ("angle"). A shape with four sides is called a quadrilateral, because it has four ("quad") sides ("lateral"). A shape with five sides is called a pentagon, because it has five ("penta") sides ("gon"). Some of these words come from Latin, and others from Ancient Greek. It would be much easier if we followed the same pattern!

Following the Greek style, we would have a trigon, a tetragon, and a pentagon
Following the Latin style, we would have a triangle, a quadrangle, and a quintangle
... or a trilateral, quadrilateral, and quintilateral

But mathematical language is just like any other language - full of inconsistencies and historical relics, developed over time by different cultures and peoples for different reasons. But what matters most of all are the geometrical concepts that these words describe, transcending both culture and language alike.

We will explore the different kinds of quadrilaterals and the families they are organised into.

The first division lies between the convex quadrilaterals and the concave quadrilaterals:

Notice that the concave quadrilaterals stick into themselves - we can form a triangle with three of their points, with the fourth one lying inside it:

The internal point has been highlighted for each concave quadrilateral.

Most of the time we will be looking at convex quadrilaterals. With one exception, all the special quadrilaterals we mention in this lesson are convex.

Trapeziums and parallelograms

If two sides of a quadrilateral are parallel, we call it a trapezium. The angles on the sides connecting the parallel lines are always supplementary, because they form a cointerior angle pair:

If the four sides of a quadrilateral form two pairs of parallel sides, we have a special kind of trapezium called a parallelogram. Here are some parallelograms:

These shapes always have all of these properties:

Opposite sides are parallel (by definition)
Consecutive angles are supplementary (because it is a trapezium in two ways)
Opposite angles are equal
Opposite sides are equal in length

If a quadrilateral has any one of these properties, it will be a parallelogram with all the other properties as well. Here are two more shapes that are also parallelograms:

Because these shapes have one of the above properties, they have all of them - including the defining one, that opposite sides are parallel.

Kites

A different kind of quadrilateral is called a kite, where the shape has two pairs of adjacent sides that are equal in length. Unlike the trapezium family, kites can be both concave and convex. Sometimes you might hear a concave kite referred to as a dart.

If a shape is a kite, it has an additional property. The angles between each pair of equal sides may be different, but the other two angles are always the same:

Rectangles, rhombuses, and squares

If a parallelogram has four equal angles, they are automatically right angles. A parallelogram with four right angles is called a rectangle, and has these properties:

All angles are right angles (by definition)
Opposite sides are parallel and equal in length (because it is a parallelogram)

Here are two rectangles:

A different kind of shape is both a parallelogram and a kite at the same time, called a rhombus, which has these properties:

Opposite sides are parallel (because it is a parallelogram)
Consecutive angles are supplementary (because it is a trapezium)
Opposite angles are equal (because it is a parallelogram)
All sides are equal in length, because:
- Opposite sides are equal in length (parallelogram)
- Two pairs of adjacent sides that are equal in length (kite)

Here are two rhombuses:

The most special kind of quadrilateral is the square. It is a combination of a rectangle and a rhombus, so it is also a parallelogram, a kite, and a trapezium! These are the properties that all squares have: