Lesson video

Oh, hi, everyone.

Almost forgot you were there.

I'm Mr. Lund and in this lesson, we're going to be finding the radius or the diameter of a circle when you're given the area.

If we know the area of a circle, how can we go on to calculate the radius? Let's have a look at this question.

In this example, the area of the circle is 452.

39 centimetre squared.

Let's work out the radius of the circle in terms of pi.

First of all, write down your formula for the area of a circle.

Then substitute the value for area into your formula.

Next, divide both sides of this formula by pi.

That would find me this fraction is equal to the radius squared.

But we haven't solved it yet.

By taking the square root of both sides, we'll find radius.

There's our answer in terms of pi.

Let's look at another example.

The area of a circle is 153.

94 centimetres squared.

Work out the diameter of the circle.

There we go.

Here's the formula for the area of a circle.

Always try to write it down first, and then substitute the value for the area into the formula.

Dividing both sides by pi, we'll find 153.

94 over pi equals the radius squared.

I can simplify my fraction, and it simplifies to the number 49.

That is equal to the radius squared.

So taking the square root of both sides finds that the radius is the square root of 49, which equals seven.

But remember, we're looking for the diameter, so we need to double our answer.

And the diameter, in this case, is 14 centimetres squared.

Here's some examples for you to try.

Pause the video and come back to check your answers.

Here are the solutions to questions one and two.

Whenever you're doing any kind of work with complex formulas, show your working out.

If you're in the exam, really show your working out.

Let's try questions three and four now.

Pause the video and return to check your answers.

Here are the solutions to questions three and four.

Again, show your workings.

Take your time and make sure that you're getting everything right before you move on.

Here's a lovely little problem solving question for you? Pause the video one more time to check your answers.

Here's the solutions to question five.

Hopefully you recognised that the diameter of the circle, which is the longest length within that circle is also the side length of the square.

And, the very last question for you.

Well done for getting this far.

Pause the video and return to check your answers.

Here's the solutions to question number six.

Be careful when you are rounding.

In your answers, you may have rounded at a different point, so you might have ended up with a slightly different answer.

In your exams, make sure that you write a little note explaining that you have answered.

Maybe you've rounded to two significant figures or two decimal places.

Make sure you make a little note of that, as you are rounding.

Okay?.