Lesson video

Hi, I'm Miss Kidd-Rossiter, and I'm really glad that you can join me today as we get started with this unit on area and perimeter of 2-D shapes.

We're going to start today by looking at describing perimeters.

So before we get started, can you make sure, please, that you're in a nice quiet place if you're able to be, you're free from all distractions, you've got something to write with, and something to write on.

If you need to pause the video now to get any of that sorted, then please do.

If not, let's get goin'.

We're starting today's lesson with a Try this activity.

How many different shapes can you draw on a centimetre square grid that have a perimeter of 12 centimetres? Pause the video here and have a go at this task.

Excellent, there are absolutely loads of shapes that you could draw here.

There's two, obviously, on the screen there that you could have drawn.

Here are some other solutions.

There are more solutions here, but I've limited my solutions to ones that are rectilinear, so that means that they have a right angle, and all vertices are on the square corners on the grid.

So, there's some solutions.

If you came up with extra ones, really well done.

The perimeter of a shape is the length of its boundary.

I want you to pause the video here and write that down, because that's really important, and it's going to be a key to everything else that we're doing this lesson.

So pause the video here and write that down.

So, some students are comparing the perimeters of A and B.

Javier says, "A is less than 4 sticks, "and B is more than 3 sticks." Bin says, "A is 300 centimetres, "and B is 250 centimetres." Yasmine says, "A is 18 hands, and B is 15 hands." And Zacky says, "A is three metres, and B is 2.

5 metres." Pause the video here and tell me what's the same and what's different about their responses.

Excellent work.

I'm sure you came up with loads there.

What's something that's the same? Tell me now.

Really good.

So something that's the same that I noticed are that Bin and Zacky are using centimetres and metres, and these are formal units.

That means that they are the same everywhere, and we can compare them, can't we? We know that 300 centimetres is equal to three metres.

Javier and Yasmine are using sticks and hands.

These are informal units, and this means that they are not the same everywhere.

It depends on the stick and the hand that you're using, and we can see that these are probably less accurate than the other measurements of centimetres and metres.

Well done there.

So, we've got here on the screen five squares and four isosceles triangles, and they've been combined.

Pause the video here and see if you can link these calculations to the perimeter.

Pause the video now.

Excellent, so if we look at the first one to start with, eight times two plus three, we know that this side here is two centimetres because isosceles triangles have two sides that are the same length, and we know that this side here is three centimetres because it's a square.

And all the sides of a square are the same length.

So that means that these two sides together would be two add three, wouldn't they? So, where does the eight times two plus three come from? Excellent, we've got eight of these pairs of sides, haven't we? One, two, three, four, five, six, seven, eight.

So we've got eight lots of the two plus three centimetres.

What about the second one, then? Four times two plus two plus three plus three? Where does that come from? Can you tell me now? Excellent.

We've got two, plus two, plus three, plus three.

That makes up this here, doesn't it? Because each of the isosceles triangles have the same length here, and then these are sides of a square, so that must be there.

So to work out this perimeter that I've highlighted, we would do two, add two, add three, add three.

So where does the four times come from? Can you tell me now? Excellent, I've got four of these, haven't I, that make up the perimeter.

One, two, three, four.

So that is where my four times that comes from.

And I mustn't forget to include my units.

Both methods are really valid.

And both methods will give you the same perimeter.

I'd like you now to pause the video and have a go at drawing shapes for these perimeter calculations that are at the bottom of your screen.

So pause the video now and have a go at that.

What did you get? There might have been different solutions to these, so the ones that I give you are not exhaustive.

And if you drew something different, that's probably okay.

So for the first one, three times six centimetres, well, I drew a triangle because I know a triangle has three sides.

And I said that each of my sides here was six centimetres.

Can you tell me what type of triangle that means that this shape is? Excellent, it's an equilateral triangle.

So that's one option.

I also could have drawn for the second one a quadrilateral that looks something like this, where I've got three sides that are five centimetres and one side that is seven centimetres.

Again, yours might've looked different, and that's absolutely fine.

And then for the final one, I drew a rectangle where I had my length is four centimetres and my width is five centimetres.

So that's the same here and here.

So that means that I can do five add four, or as it's written in the calculation, four add five.

And then I've got two pairs of those sides, so I can multiply it by two and get my answer.

You're going to pause the video now, navigate to the independent task, and give that a go.

When you're ready, resume the video.

Excellent work on that independent task.

Well done for persevering with it.

Let's go through the answers.

So for question one and two, the answers are now on the screen.

Pause the video here to check your work.

For the third question, then, here are your answers.

Check these.

Excellent work, well done.

So we're moving on to the Explore task, now.

The grid below uses nine identical rectangles, and we've got the measurement of one of those rectangles there.

What is the perimeter of this shape? Explore the effect on the perimeter when you remove rectangles.

So there's one example in the bottom corner of when we remove one rectangle.

What about if we remove more than one? What about if we're not removing them from the corner? Pause the video here, and have a go at this task.

Excellent work, then.

What did you get for the perimeter of the shape? Tell me now.

Excellent.

So you could have said several different things here.

You could have said that the perimeter is 6a, add 6b metres.

So that's all in metres.

Or you could have said the perimeter is two times 3a plus 3b metres.

Or you might have written it differently again.

I like this bottom one because it means I only have to work out two sides.

So a, add a, add a, add b, add b, add b.

This side would be 3b.

This side would be 3a, and I know that I've got two pairs of those sides.

What about this one, then? Explore the effect on the perimeter when rectangles are removed.

So if we removed one rectangle here in the corner, what happens? Tell me now.

Excellent, no change to the perimeter.

If we're removing one or two rectangles from the corner, then the perimeter remains unchanged.

Excellent.

What about if we remove a whole row or a whole column? So if we just looked at this shape here, what would that be? What happens then to the perimeter? Excellent, it reduces doesn't it? So if we remove a whole row or a whole column, then we are reducing the perimeter.

What happens, for example, if we remove, let's say, this rectangle here? So we're looking at the perimeter that I'm drawing on now of the red shape.

What happens there? Can you tell me? Excellent.

In this case the perimeter is increased, isn't it? Because we still have 3a, add 3b, add 3a, add 3b, but then we have to add on another two a's to the perimeter.

Excellent work there.

Well done.

That's the end of today's lesson.

So thank you so much for all your hard work.

Please don't forget to go and take the end-of-lesson quiz, and hopefully I'll see you again soon.

Bye.