NCERT Book Class 7 Maths Perimeter and Area

Perimeter and Area

11.1 INTRODUCTION

In Class VI, you have already learnt perimeters of plane figures and areas of squares and rectangles. Perimeter is the distance around a closed figure while area is the part of plane or region occupied by the closed figure. In this class, you will learn about perimeters and areas of a few more plane figures.

11.2 SQUARES AND RECTANGLES

Ayush and Deeksha made pictures. Ayush made his picture on a rectangular sheet of length 60 cm and breadth 20 cm while Deeksha made hers on a rectangular sheet of length 40 cm and breadth 35 cm. Both these pictures have to be separately framed and laminated.

Who has to pay more for framing, if the cost of framing is Rs 3.00 per cm?

If the cost of lamination is Rs 2.00 per cm2, who has to pay more for lamination?

For finding the cost of framing, we need to find perimeter and then multiply it by the rate for framing. For finding the cost of lamination, we need to find area and then multiply it by the rate for lamination.

TRY THESE

What would you need to find, area or perimeter, to answer the following?

1. How much space does a blackboard occupy?

2. What is the length of a wire required to fence a rectangular flower bed?

3. What distance would you cover by taking two rounds of a triangular park?

4. How much plastic sheet do you need to cover a rectangular swimming pool?

Do you remember,

Perimeter of a regular polygon = number of sides × length of one side

Perimeter of a square = 4 × side

Perimeter of a rectangle = 2 × (l + b)

Area of a rectangle = l × b, Area of a square = side × side

Tanya needed a square of side 4 cm for completing a collage. She had a rectangular sheet of length 28 cm and breadth 21 cm (Fig 11. 1). She cuts off a square of side 4 cm from the rectangular sheet. Her friend saw the remaining sheet (Fig 11.2) and asked Tanya, “Has the perimeter of the sheet increased or decreased now?”

Has the total length of side AD increased after cutting off the square?

Has the area increased or decreased?

Tanya cuts off one more square from the opposite side (Fig 11.3).

Will the perimeter of the remaining sheet increase further?

Will the area increase or decrease further?

So, what can we infer from this?

It is clear that the increase of perimeter need not lead to increase in area.

Please refer to attached file for NCERT Class 7 Maths Perimeter and Area