Table of Contents

Last modified on April 22nd, 2021

Based on their sides, all quadrilaterals can be broadly classified into two different groups: regular and irregular.

A regular quadrilateral is a type of quadrilateral with four sides of equal length and four angles of equal measure.

**Example**: A square is the only regular quadrilateral

An irregular quadrilateral is a type of quadrilateral having one or more sides of unequal length and one or more angles of unequal measure.

**Examples**: Rectangle, parallelogram, rhombus, trapezoid, and kite are irregular quadrilaterals.

A square is the only regular quadrilateral known to us. Thus, to find the area of a regular quadrilateral we will use the formula for determining the area of a square. The formula is given below:

Formula

**Area (A) = a ^{2}**, here a = side

In regular quadrilateral ABCD,

a = AB = BC = CD = DA

**Find the area of the regular quadrilateral ABCD whose sides measure 12 m.**

Solution:

As we know,**Area ( A) = a^{2}**, here

= 12 x 12 m

= 144 m

Since all irregular quadrilaterals are different in shape, we cannot apply the formula of any particular quadrilateral to find the area of all irregular quadrilaterals. In other words, we do not have a fixed, general formula that can be used for all of them. In such cases the following steps are followed:

- The quadrilateral is divided into two triangles by drawing a diagonal
- Find the area of each triangle using different formulas as described in our triangles article
- Add the areas of two triangles

The steps though sounds easy can sometimes be quite elaborate. Let us take an example to understand the concepts better.

**Find the area of an irregular quadrilateral ABCD when sides BC = 6cm, CD = 8 cm, DA = 10 cm, and AB = 12 cm and ∠BCD = 120°.**

Solution:

Let us divide the irregular quadrilateral ABCD into △BCD and △DAB by drawing a diagonal BD

Since we do not know the height of any of the triangles BCD and DAB, we cannot use the general formula **½ x base x height**, for determining the area of triangles.**Using Side-Angle-Side (SAS) Law**

Here, we will use the trigonometric function (**A**) = **½ x BC x CD x sin C**, to calculate the area of △BCD

A = ½ x 6 x 8 x sin 120°

= ½ x 48 x √3/2 cm^{2}

= 20.78 cm^{2}**Using the Law of Cosines**

We now know the area of △ BCD, but still do not know the length of the diagonal BD. For that, we will use the **Law of Cosines**:**c ^{2} = a^{2} + b^{2} – 2ab cos C**

In △BCD, let BC = h, CD = a, and BD = t

Then the above law of Cosines can be written as,

t

t

t

t

t

t

t = 12.165 cm

We now have the approximate length of the diagonal BD as 12.165 cm of the irregular quadrilateral ABCD.

Knowing the diagonal of the quadrilateral ABCD, we can now calculate the area of the other section of the quadrilateral using the Heron’s formula.

Since Heron’s formula depends on knowing the semi-perimeter, for △DAB, the three sides are:

DA = 10 cm, AB = 12 cm, and BD = 12.165 cm

As we know, semi-perimeter (

= 34.165/2 cm

= 17.08 cm

Now, applying Heron’s Formula to calculate the area of △DAB,

= √17.08 x (17.08 – 10) x (17.08 – 12.165) x (17.08 – 12) cm

= √17.08 x 7.08 x 4. 915 x 5.08 cm

= √3019.314 cm

= 54.948 cm

Now, to obtain the area of the irregular quadrilateral ABCD, we need to add the area of the triangles BCD and DAB

Hence, Area of the irregular quadrilateral ABCD = Area of △BCD + Area of △DAB

= (20.78 + 54.948) cm

= 75.728 cm

**Ans**. There is no such quadrilateral that which has four congruent sides but is irregular because the only quadrilateral that has four congruent sides is a square, which is a regular quadrilateral.

**Ans**. Rhombus

**Ans**. All four sides of a trapezoid are not equal and hence it is not a regular quadrilateral.

**Ans**. We usually call a regular quadrilateral, a square.

**Ans**. Square

**Ans**. The square being the only regular quadrilateral, it has four lines of symmetry.

**Ans**. A regular quadrilateral has four sides.

**Ans**. Since a rectangle has only opposite sides equal, they are not a regular quadrilateral.

**Ans**. Since a rhombus does not have all its four angles equal, they are not a regular quadrilateral.

**Ans**. Rectangle.

**Ans**. Since, a concave quadrilateral has one of the interior angles measuring more than 90°, it is not possible to have a quadrilateral that is concave but regular.

**Ans**. The only irregular quadrilateral that has right angles is a rectangle.

Last modified on April 22nd, 2021