How_To_Be_Good_At_Math

MEASUREMENT • PERIMETER 165

The field’s corners are right

angles, so its opposite sides are

parallel to each other. That means

if we know the length of one side,

we can work out the length of the

unknown part of the opposite side.

Let’s find the missing length.

The opposite side is 12 m long,

so the two sides facing it must also

have a total length of 12 m.

To find the missing length, we

just need to subtract 9 from 12:

12 − 9 = 3. So, the length of the

missing side is 3 m.

Now we can work out the

total perimeter by adding

up the lengths of all the sides:

12 + 6 + 9 + 5 + 3 + 11 = 46

This means that the perimeter

of the field is 46 m.

Look at this field. We need to

find the perimeter but the length

of one side is missing.

What if we don’t know

the lengths of all the sides?

Sometimes we don’t know the lengths of all the sides of

a shape. If a shape made up of one or more rectangles

has a measurement missing, we can still figure out the

missing length and the perimeter.

6 m

5 m

9 m

12 m

11 m

?

164_165_Perimeter.indd 165 29/02/2016 18:03