Lesson video

Hi, I'm miss Kidd-Rossiter and I'm going to be taking the lesson today on describing enlargements.

Before we get started, can you please make sure that you're in a nice, quiet area, free from distractions, and that you're ready to concentrate on this lesson.

If you need to pause the video now to get any equipment or to find yourself a quiet space, then please do, if not, let's get going.

We're starting today's lesson with the try this activity, you've got three students on the screen, Been, Carla and Japhia who have given you three descriptions of the enlargement, which is shown on the grid.

Your job is to figure out what's missing from each people's instructions.

Pause the video now and have a go at this activity, when you're ready, resume the video.

You'll see the three students are still on the screen and we've got their three descriptions there too.

When we're trying to figure out what's missing from each pupil's instructions, the things that we need to remember are, that when you're describing an enlargement you must state, what are you doing? What is the scale factor? And where is the centre? So let's first look at Been's description, she says scale factor three, centre one zero.

So we can quite clearly see that she's given us the scale factor and she's given us the centre, but she has not told us what are we actually doing? If you haven't already, pause the video now and think about what's missing from Carla and Japhia's instructions.

Carla says shape A has been enlarged, so she's told us what we're doing, and she said, it's by scale factor of three, so she's told us what the scale factor is, however, she hasn't told us where the centre is, so hers is incomplete.

Japhia has said shape A has been enlarged, so again, he's told us what we're doing, but, and he has said it's about centre one zero, but again, he has not told us the scale factor so there's something missing from his explanation.

Pause the video now and see if you can come up with a perfect instruction to describe this enlargement.

Brilliant, can you read out to the screen please? Here's my own set, I said, shape A has been enlarged by scale factor three, the centre of enlargement is one zero.

So I've told us what we're doing, enlargement, I've said what the scale factor is, and I've said the centre.

So, as long as your description has all three of those things, it's going to be brilliant.

Okay, so we're going to start the connect activity now.

So what we're going to do is describe some enlargements.

First of all, we're going to look at describing the enlargements of the triangles on the right hand side of your screen if the blue triangle is the object, so that's this triangle here.

This is our object and first of all, we're going to think if my black triangle is the image.

So to find the centre of enlargement, what we would do is we would draw on our ray lines and see where they meet.

So we draw through the corresponding vertices of each of the two shapes and see where they meet.

Now, clearly on this one, I'm giving this P and I'm not really good at doing this free hand, so I've done it and I'll show you what it should look like.

So we can see here that our centre of enlargement is P.

Now we need to find the scale factor.

So from P to this Vertex here on my blue triangle, it's two squares, and from the same centre of enlargement to the same Vertex on the black triangle, it's four squares.

So that would tell us that our scale factor of enlargement is two but remember for an enlargement it has to be the same scale factor applied to each of the sides, so let's just double check it's the same.

So from P to this Vertex here is two squares and from P to this Vertex here is four squares, so we can be quite confident that the scale factor of enlargement from the blue triangle to the black triangle is two.

So if we were to write that as a sentence we would write, the blue triangle has been enlarged by scale factor two about centre P, to give the black triangle.

What I want you to do now is pause the video and have a think about what would happen if the blue triangle is the object and the red triangle is the image.

So hopefully you would have realised that we can just extend the ray lines to go through the vertices of the red triangle, you would also do it with a ruler.

Then you should have written the following statement, the blue triangle has been enlarged by scale factor three about centre P, to give the red triangle.

What I'd like you to do now is have a go at describing the enlargements if the red triangle is the object this time.

So we've got the red triangle is the object, and let's do, first of all, that the black triangle is the image.

What do you need to do first of all? What do you need to do then? Write your sentence.

Pause the video and then have a go.

Hopefully you should have got this statement, so the red triangle has been enlarged by scale factor two thirds about centre P, to get the black triangle.

Now I'd like you to have a go if the red triangle is the object and the blue triangle is the image.

Pause the video now and have a think about this.

Okay, you should have got, the red triangle has been enlarged by scale factor one third about centre P, to give the blue triangle.

If you need a little bit more practise, pause the video now and have a go at this one, I'll put the answers on shortly, if not navigate to the independent task now and have a go at applying your learning.

And here are the answers for this one, hopefully that's what you got.

Now what I'd like you to do is pause the video, navigate to the independent tasks to apply your learning, and then when you're ready resume the video.

So we're going to go through the answers to the independent task now, what I'm going to do is I'm going to put the answers on the screen and I'll leave them up for about three seconds.

If you need longer, just pause the video each point to mark your work.

So here's the answer to question one.

You'll also see that I've added the ray lines to foresee how I found the centre of enlargement.

Question two is now on the screen.

Question three for the first part is now on the screen and question three for the second part is now on the screen.

Right, finally, we're moving on to the explore task.

So you've got two students here and they're both giving you a statements, pause the video now and see which student do you agree with and why? So did you come to a decision? Who do you agree with? I'm going to draw some ray lines onto the diagram and see if you can clarify your decision further.

Here's the first ray line, I don't think I can find the centre of enlargement yet, do you? It's a secondary ray line, I might have an idea now about where my centre of enlargement is, it could be two seven, but I'm going to draw a third ray line on just to make sure.

Oh, okay, drawing my third ray line on, looks like it's definitely two seven.

Let's just double check and triple check with our final ray line.

Yes, I can see now that my centre of enlargement is definitely two seven, so who do you agree with? Can we use just two corresponding points or do we need to use all of them? I'm going to leave that with you to think about.

And on that note, that's the end of today's lesson.

I hope you've learned a lot, I've really enjoyed taking you through it, I hope to see you again soon, bye.