How_To_Be_Good_At_Math

perimeter Using formulas to find

For a rectangle, we can add

up the two lengths that are

different then multiply by two,

because there are two sides of

each length. We use the formula:

We know that all four sides

of a square are the same

length. We can find the

perimeter by adding those

four sides together.

Just like a rectangle, a

parallelogram has two

pairs of opposite sides that

are parallel and equal in length.

Look at this red square.

If we call the length of each

side “a”, we can say

Perimeter = a + a + a + a.

A simpler way of writing this is:

So, we can use the same

formula for a parallelogram

as for a rectangle, adding the

two adjacent side lengths

together then multiplying by two:

Let’s imagine that the

square’s four sides were

each 2 cm long. The perimeter

would be 8 cm, because 4 × 2 = 8

So, if the rectangle’s sides

were 2 cm and 4 cm long,

the perimeter would be 12 cm,

because 2 (4 + 2) = 12

This means that if the sides

were 3 cm and 5 cm, the

perimeter would be 16 cm,

because 2 (5 + 3) = 16

A rectangle has two pairs

of opposite sides that are

parallel and equal in length. Let’s

call the length in one pair “a” and

the length in the other pair “b”.

Using formulas to find

perimeter

If we remember some basic facts about 2D shapes, we can

use formulas to find their perimeters. These formulas use letters

to represent the lengths of the sides. This makes it easier for us to

remember how to calculate the perimeters of lots of different shapes.

Square Rectangle Parallelogram

a a

a

a

b b

a a

a a

b b

Perimeter of

a square = 4a

Perimeter of

a rectangle

Perimeter of

= 2 (a + b) a parallelogram = 2 (a + b)

166_167_Using_formulas_to_find_perimeter.indd 166 29/02/2016 18:03