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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