## Revision Notes ofÂ Chapter 12 Heron's Formula Class 9th Math

**Topics in the Chapter**

- Basics Revision
- Area of Triangle
- Heron's Formula
- Area of equilateral Triangle

**Basics Revision**

â€¢ Area of a triangle =Â 1/2 Ã— Base Ã— Height

â€¢

__Heronâ€™s formula:__For sides of a triangle being a, b, c:

**Area of Triangle =Â**

where s is semi-perimeter and

**s = (a+b+c)/2**

â€¢ For finding area of a quadrilateral we divide it into various triangles. Then we use Heronâ€™s formula to find the area of the triangles.

**Area of Triangles**

= 1/2 Ã— (side) Ã— [altitude corresponding to that side (or height)]

= 1/2 Ã— BC Ã— AD

= 1/2 Ã— a Ã— h

Thus, area of a triangle = 1/2 Ã— Base Ã— Height

__Note:__

Unit of measurement for area of any plane figure is taken as square metre (m

^{2}) or square centimetre (cm

^{2}), etc.

**Area of TriangleÂ using Heron's Formula**

When it is not possible to find the height of the triangle easily and measures of all the three sides are known then we use Heronâ€™s formula, which is given by:

Area of a triangle =Â

where a, b and c are the sides of the triangle and s = semi-perimeter, i.e. half the perimeter of the triangle = (a+b+c)/2

**Area of an Equilateral Triangle**

Let the side of the equilateral triangle be â€˜aâ€™.

^{2}