 Hong Kong
Stage 1 - Stage 3

# Perimeter I

Lesson

## Perimeter

The perimeter of a shape is the total distance around the outside. The perimeter of any polygon is found by adding up the length of each side. To work out the perimeter of a triangle, there are some cases where we only need the length of 1 or 2 sides.

Rectangles have some special features, so working out the perimeter of a rectangle may involve a little less work! If we have a shape made up of more than one rectangle, we have a composite rectangle.

### The perimeter of a composite shape

The shapes we're looking at here might look a little more complicated, but by imagining them as rectangles joined together, we can work out the length of missing sides. This video shows you how, as well as how to work out the perimeter, even if the length of each side is not given.

#### Worked Examples

##### Question 1

Find the perimeter of the shape. ##### Question 2

Find the perimeter of the shape.

 $13\frac{1}{2}$1312​ cm $6$6 cm $5\frac{1}{2}$512​ cm $13$13 cm $7$7 cm $8$8 cm

## The perimeter of regular shapes

When we have a regular shape, every side has the same length, so working out the perimeter is the same as multiplying the length of one side by the number of sides we have. How neat is that? Here's a video to show you just how handy this is.

#### Worked example

##### Question 3

If an equilateral triangle has a perimeter of $30$30 mm, how long is each side?

Remember!

Even though some shapes have features that mean we don't need to know each side length to work out their perimeter, you can still find the perimeter of a shape by adding up the length of each side.