# Introduction

In earlier classes, we have learnt to calculate the area and perimeter of rectangles, triangles, squares and quadrilaterals.Â

We have even looked at the practical application of such calculations.Â

The unit of measurement for length and breadth as metres (m) or centimetres (cm)

The unit for area would be in square metres (m^{2}) or square centimetres (cm^{2}) etc.

Area of a triangle = Ã— base Ã— height

In the case of a right angled triangle, the formula is applied using the two sides containing the right angle as base and height.

Hence if we have a right angled triangle with the sides containing the right angle of length l and b then the area is given by

Area of a right angled triangle =

Ã— l Ã— bArea of a triangle =

Ã— base Ã— height

Area of Equilateral TriangleÂ

Â

Area of Î” ABC = Â´ base Ã— height

For an equilateral triangle of side a ,

If AD is perpendicular from A to BC, then AD bisects BC, i.e., BD = .

Â

In right Î” ABD , AB^{2}Â = AD^{2} + BD^{2} , AD^{2} = AB^{2} - BD^{2 }= Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â

Â Â Â Â Â Â Â Â Â Â Â Â Â Â ADÂ =

Â Area of Î” ABC =

In the case of an isosceles triangle we can use the formula thus. Let us take a triangle ABC with two equal sides AB and AC of length = 5cm and the unequal side equal as 8 cm. To calculate the height of the triangle we draw the perpendicular AD from A onto BC. The perpendicular AD divides the base BC into two equal parts.

Â

Â

Find the area of a triangle in which base = 2m and height = 125 cm.

BaseÂ ofÂ theÂ triangleÂ =Â 2mÂ =Â 200cm

Area of theÂ triangle = Ã— base Ã— height

Â Â Â Â Â Â = Ã— 200 Ã— 125

Â Â Â Â Â Â = 12500 cm^{2
Â Â Â Â Â Â Â }= 1.25 m^{2}