Lesson video

Hi everyone.

I'm Mr. Lund and in this lesson, we're going to find the diameter or the radius of a circle, given the circumference.

Hi, everyone.

In this question, we're told the circumference of a circle is 23.

3 centimetres.

We're asked to work out the diameter of the circle in terms of pi.

Here's our circle.

We're looking for the diameter.

Let's start with the formula to find the circumference.

Circumference equals pi times by diameter.

You have to memorise this for your exam.

If I substitute the value of 23.

3 into our formula, here's what I'd get.

Pi is just a number, so let's divide both sides of this formula by pi.

There we go, 23.

3 over pi is equal to the diameter.

Let's put that on our diagram.

There we go.

Here's some questions for you to try.

Pause to video and return to check your answers.

Here's the solutions to question one and two.

If you didn't get the right answers, go back and check until you do get the right answers.

It's important for the rest of the lesson.

If you can work out the diameter of a circle, when given the circumference, it's easy to work out the radius.

Here's a circle.

We want to work out the radius.

Here's our formula, circumference equals pi times by diameter.

Let's substitute the value of 40 into our formula.

Divide both sides by pi, finds 40 divided by pi equals the diameter.

The diameter is 12.

732, et cetera.

The radius is going to be half that value.

So dividing by two, finds the radius.

There's our answer to three significant figures.

Here's some questions for you to try.

Pause the video and return to check your answers.

Here are the solutions to questions three.

When you are rounding, communicate where you have rounded your answers, especially in exams. It can alter the answers you get.

It's easy to make mistakes with more complex calculations.

Can you spot the mistakes here? Pause the video and return to check your answers.

Here are the solutions to question four.

We all make mistakes with mathematics.

Being able to spot those mistakes is part of learning.

Well done for getting this far.

Pause the video and returned to check your answers.

Well done for getting this far.

Here, you have a larger circle overlapping a smaller circle.

It might help to draw the larger circle and the smaller circle separately on a piece of paper to help you with your calculations.