Find the Perimeter of Part Circles

In this activity, we will find the perimeter of shapes which have parts of circles in them. 

In order to do this, we have to be able to calculate the circumference of a circle.

You will need a calculator for this!

Recap - The Circumference

Circumference = π x diameter

C = π d

As the radius is half of the diameter we can also use:-

Circumference = π x 2 x radius

C = 2 π r

Write these down because we will use them for the exercise!

Let's try a question using this!

Example

Find the perimeter of the semicircle below.

Give your answer to 1 decimal place.

Answer

First, we need to find the circumference of the full circle and divide by 2, because we only want half of it!

The diameter = 10 cm

Circumference  = π x diameter ÷ 2

=  π × 10 ÷ 2 

Use your calculator button π =  15.70796...........

We need to give our answer to 1 decimal place

= 15.7 cm

15.7 is the arc (half the circumference) 

We want the perimeter!

To finish the perimeter we must add the 10 cm of the base of the shape.

Perimeter = 15.7 + 10 = 25.7 cm

Let's try some questions!