Lesson video

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Hello everybody, my name is Miss Brinkworth.

I'm going to be going through this math lesson with you today.

So if we look at our learning objective, what we're going to be doing is calculating the perimeter of simple compound shapes.

So we're going to use what we already know about perimeter in terms of rectangles and apply that to shapes, which are made up of rectangles.

So let's go through what the agenda's going to look like.

So we're going to have a quick look at what we mean by compound shapes.

We're then going to think about how we find missing values.

So where we are being asked to find the perimeter of a shape where maybe it looks like we're not given every single line and every single side measurement.

And then you've got an independent task where you get to apply those skills and there'll be a quiz at the end for you to have a go at seeing how well today's learning has gone in.

So, you will need pencil, ruler, paper.

If you can't find a ruler because you're at home, please don't worry too much.

If you can it will be quite useful, but there will be everything on the screen that you need.

So take your time to gather your equipment and come back when you're ready.

Well done, welcome back, let's get started then.

What is a composite shape? Well, a composite shape is one which is made up of two or more shapes pushed together to make one.

So, you probably feel quite confident working out the perimeter of rectangles and squares.

These shapes are not rectangles and squares, but they are made up of rectangles and squares.

They have been pushed together so we can split them up and work out the perimeter of the ones that we know and then we can put that together to work out the perimeter of the whole shape.

So what shapes can you spot here? Where do you think you would draw some lines to split these shapes up into ones that you would feel confident working out the perimeter of? Well, there's some different options really.

You could draw a line here couldn't you on this shape and now you've got two rectangles.

or you might want to draw the line here and then it looks like you've made a square and a rectangle maybe that would be easier to work out the full perimeter.

In this other shape, we could split it like this into three rectangles, or again, there's another option we could split it at the top there, maybe that gives us a square and one rectangle, so we've just got two shapes to work out the perimeter of.

So today some of it will come down to you deciding how you would like to split these shapes up in the most useful way so that you can work out the perimeter.

Okay, how does splitting the shape actually help then? Well, it helps because we know how to calculate the perimeter of rectangles and squares so if we split a shape up into rectangles and squares, we can work out those perimeters and then when we add those together, clearly we can work out the perimeter of the whole shape.

Okay, how would I work out the perimeter then for a shape where I don't have all the measurements? So look at the shape here.

Firstly, think about where would you like to put some lines to split the shape into two shapes that you feel confident working out the perimeter of? Well, I am going to put the line here.

I'm going to cut off that a bit at the top into a smaller rectangle and then I've got the large rectangle at the bottom.

I now just need to work out the perimeter of the whole shape.

So, how am I going to do that? Well, how do I know that that side there is two centimetres? How do you think I've been able to work out that that side is two centimetres? Well, I've done it because you can see that if you're comparing the one down the side that says four, and then the longer one that says six, the difference between four and six is two and that little line is the difference between those two lines, so it must be two centimetres.

How do I know that that one's four? Well, the line along the bottom is eight and the one at the top is four, so the difference between eight and four, what have I got to add to four to get that one at eight, I've got to add four.

So I've worked out my missing sides through some subtraction and using what I've been told.

It might not always look like it, but you will have been given all the information you need to work out those sides you just need to think carefully about how to work them out.

And normally you can do it by adding or subtracting from the sides you've been given.

I can now add all the sides together to work out the perimeter.

Okay, your turn.

Have a go at splitting the shapes and decide where that line, or lines, is going to go.

Make sure you know the perimeter, the length of every side, so you're counting the measurement of every side and then add them together to find the perimeter.

Okay, let's see how you did.

Here's a good place to put those lines and then we can work out that this one is two.

I know that that's two, because I've got five and two is seven, but the one at the bottom is nine so if I've got seven already, I need two more to get me to nine.

And this one, it must be one 'cause it's the same as the one opposite it and we know that adjacent sides on rectangles are the same, so that one must be one as well.

Are there any others I need to write in do you think? We'll know that that one's three 'cause the other one on the side is three and that forms a rectangle so they must be same as each other.

You've got to make sure that you go all the way around the perimeter of the shape, making sure you have a measurement for every single side.

I can then add them all together to find the perimeter of the whole shape.

Okay, it is time now for your independent task.

Just before it I'm going to take you back a slide and just remind you what we are doing here.

So you're going to see a shape which is made up of rectangles or squares, decide where you want to split that shape up, to turn it into a series of rectangles, then work out any missing sides.

So go around every single side of the shape and work out.

If a side hasn't been given a measurement you can work it out, you can work it out from the side you've been given.

They'll either be a side opposite it, or adjacent to it, opposite it, sorry, which will be the same, or you can work out from the sides you've been given that that one is the missing bit.

So like on this one for example, the bottom of my rectangle is nine, then I've got five across the top and two across the top, so to find that missing one, it must be two because if I'm getting nine, I must get five and two is seven, two more is nine.

So, take your time because there's quite a lot to do here.

Once you've worked out every single side, add all of them together to give you the perimeter for the whole shape.

I would probably want to use some common addition to make sure I've added together the sides accurately.

Okay, pause the video here take as long as you need with your independent task, come back when you finished and we'll talk about your answers.

Really well done for having a go at your independent task everybody.

There's a lot of learning going on here and for each question you have to apply a lot of different knowledge, so really, really well done for having a go.

Now, here are the answers for the perimeter for all of these shapes.

Some are harder than others, but really well done for having a go.

For each shape you needed to split it up, you needed to work out missing sides, and then you needed to add all of those together to give you your perimeter.

If you made a mistake at any stage, your perimeter might be slightly wrong so don't feel disheartened if you have made a mistake here, this is a tricky thing to do because you have to combine lots of different skills.

But if you managed to get a few of them, right, or if you're near, really well done everybody.

Like I say, these are not easy and you're working incredibly hard.

I'll make these big so you can see how you got on.

Well done today, guys, for applying what you already know about perimeter to some new questions, pulling in lots of knowledge about what you know about rectangles and how opposite sides equal.

You would have to have used some multiplication, some addition, and some subtraction work today alongside your knowledge of perimeter, so really, really good work.

We would love to see your working out so if you'd like to please ask a parent or carer to share your work on Instagram, Facebook, or Twitter, tagging @OakNational and hashtag LearnwithOak.

There's a quiz now so please have a go and see how you well you got on with today's learning.

Amazing work today everybody, really well done.

Enjoy the rest of your day.