Heron’s Formula Class 9 Notes: Chapter 12

CBSE Class 9 Maths Heron’s Formula Notes:-Download PDF Here

In Geometry, a triangle is a closed three-dimensional figure. In this article, we are going to discuss the Heron’s formula for class 9, which is used to find the area of triangles. Also, here we are going to discuss how Heron’s formula is used to find the area of other polygons in detail.


  The plane closed figure, with three sides and three angles is called as a triangle.

Types of triangles:

Based  on sides –  a) Equilateral b) Isosceles c) Scalene
Based  on angles – a) Acute angled triangle b) Right-angled triangle c) Obtuse angled triangle

Area of a triangle


In case of equilateral and isosceles triangles, if the length of the sides of triangles are given then,
we use Pythagoras theorem in order to find the height of a triangle.

Area of an equilateral triangle

Consider an equilateral ΔABC, with each side as a unit. Let AO be the perpendicular bisector of BC. In order to derive the formula for the area of an equilateral triangle, we need to find height AO.

Heron’s Formula Class 9-1

Using Pythagoras theorem,
Substitute AC=a,OC= a/2 to find OA

We know the area of the triangle is

Area of Equilateral triangle=√3a2/4

Area of an isosceles triangle

Consider an isosceles ΔABC with equal sides as a units and base as b unit.

Heron’s Formula Class 9-2

Isosceles triangle ABC

The height of the triangle can be found by Pythagoras’ Theorem :
h2=a2− (b2/4) = (4a2b2)/4

h=(1/2) √(4a2b2)
Area of triangle is A=(1/2)bh


Area of a triangle – By Heron’s formula

Area of a ΔABC, given sides a, b, c  by Heron’s formula (Also known as Hero’s Formula) :

Heron’s Formula Class 9-3

Triangle ABC

Find semi perimeter (s ) (a+b+c)/2


This formula is helpful to find the area of a scalene triangle, given the lengths of all its sides.

Area of any polygon – By Heron’s formula

To find the area of a quadrilateral, when one of its diagonal value and the sides are given, the area can be calculated by splitting the given quadrilateral into two triangles and use the Heron’s formula.

Example :A park, in the shape of a quadrilateral ABCD, has C=90, AB = 9 cm, BC = 12 cm, CD = 5 cm and AD = 8 cm. How much area does it occupy?

We draw the figure according to the information given.

Heron’s Formula Class 9-4

The figure can be split into 2 triangles ΔBCD and ΔABD
From ΔBCD, we can find BD (Using Pythagoras’ Theorem)
Semi-perimeter for ΔBCD S1(12+5+13)/2 = 15

Semi-perimeter ΔABD S2 (9+8+13)/2 = 15

Using Heron’s formula we find A1 and A2


A1= √(15×3×10×2 )
A1=√900 = 30cm2
Similarly, we find A2 to be 35.49cm2.
The area of the quadrilateral ABCD=A1+A2=65.49 cm2


  1. superb byju’s

  2. thanks for all this

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