W8 l1

PERIMETER

PERIMETER

The perimeter of a shape is the

distance all the way around its

edges.

PERIMETER

The measurements needed to

calculate a perimeter depend

on the shape and there are

many different shapes.

PERIMETER

In order to find the perimeter of a

rectangle you must first find the

length and the width.

?

?

PERIMETER

Length is usually the longest side of

a rectangle

PERIMETER

Width is usually the shortest side of

a rectangle

PERIMETER

Lets look at a few and see if it’s the

width or the length.

If its width put your finger on your

nose.

If its length put your hand on your

head.

PERIMETER

Example 1: the diagram

represents a pen for Jim’s pigs.

How much fencing does he need

to go around the plot?

4

m

4m

5m

5m

Here the length is 5m and the

width is 4m.

The perimeter of the plot is

5+4+5+4 =18.

So he needs 18m of fencing.

4

m4m

5m

5m

Example 2: The diagram

represents a pond that needs a

low railing round the boundary.

Sally is working out the perimeter.

80cm

1.2m

Caution: all measurements used in

the calculation must be the same

units.

So we know that 100cm = 1m

There are 2 zeros so we move

the decimal point 2 places to the

left.

80cm becomes 0.8m

80cm

1.2m

Caution: all measurements used in

the calculation must be the same

units.

0.8m

Now that our numbers all have the same

units we can add.

1.2m

0.8m

1.2 + 0.8 +1.2 + 0.8 = 4.0

So the perimeter is 4m. Don’t forget

to label your answers!

RECTANGLES AND SQUARES

In a rectangle the opposite sides are equal,

so to work out the perimeter of a rectangle

you just need to know the length and width.

15cm

6cm

What is the length and what is the width?

PERIMETER

There are three different methods to finding

the perimeter.

Method 1: add all the numbers together.

15cm

6cm 6cm

15cm

15 + 6 + 15 + 6 = 42cm

PERIMETER Method 2: Because opposite sides are equal

you can also work out the perimeter in this

way: double the length, double the width,

then add the results together.

15cm

6cm 6cm

15cm

(15 X 2) + 6 X 2) = 30 + 12 = 42cm

PERIMETER Method 3: The last method just adds the

length and width then doubles it.

15cm

6cm 6cm

15cm

15 + 6 = 21 21 X 2 = 42cm

PERIMETER The method you choose is up to you- each

one will give the same answer.

Lets see what you have learned. Find the

answer to each question.

Once you have found the answer put you

hands on you head like this so I know your

finished.

PERIMETER

18m

6m

6 + 6 + 18 + 18 = 48 m

PERIMETER

8cm 8cm

10cm

8 + 8 + 10 = 26cm

PERIMETER

A square

10m

10 X 4 = 40 m

PERIMETER

Homework page 59