Category Archives: enlargement

Just follow these steps and you’ll be OK at Enlargements

Here’s a statement which I don’t think will be too controversial – I would have thought maths teachers the world over would agree with this:

I want my students to gain a deep understanding of the mathematics, not just follow a procedure to get the right answer.

This is our aim. We don’t always get there. We have different ways of getting there. I have recently re-read this seminal paper by Skemp from 1976.

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In it he talks about Relational Understanding, which I have generally thought of as understanding of concepts, and Instrumental Understanding, which I think of as understanding of procedures.  In my teaching I have been inclined to build conceptual understanding first, and then see what methods make sense from there.  However, I’m starting to think that it’s not that simple.  There are some situations where some instrumental understanding might come first and act as a foundation on which to build relational understanding.  Ultimately we want both, but the order in which we achieve this is not always the same.  We should not dismiss a didactic approach that provides a clear sequence of steps and worked examples as a part of the journey to a deeper mathematical understanding.

I recently observed a colleague teaching the topic of Enlargements to a Year 11 revision class, who are entered for the Foundation paper.  He used a method which I don’t think I had thought of before.  It was heavy on the Instrumental Understanding, but it worked and the students were doing some more tricky fractional and negative enlargements with centres of enlargement not at the origin.  About as hard as it gets for these types of exam questions (thanks for Maths Genie for these examples)

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So, what was this wonderful method?  Well, it might be nothing new to you dear reader, but it used vectors.  It relied on students being secure with describing the translation between two points using vector notation.  Given that this is how Translations are described and that we typically teach Translations (along with Reflections and Rotations) before Enlargements in a topic called Transformations, this should be a build on / consolidating of what was learned a few lessons ago.

The steps go something like this.

  1. Label the vertices of the shape you’ve been given (say A,B,C,D, something like that)
  2. Circle the centre of enlargement, CoL (helpful to distinguish it from a vertex later)

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3. Find the vector that moves you from the CoL to each vertex.
4. Multiply each vector by the scale factor

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5. Now use those vectors to plot the new points, starting from the CoL again. Connect the vertices to form the shape.

6. Finally draw in some ray lines to convince yourself that you have not made any mistakes.

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I think that’s quite a neat method that enables us to go straight to what might be seen as the most difficult example. The only real difficulty here, however is multiplying a (pretty simple) negative fraction by an integer.  Something which should be secure by the time this topic is being taught. And if it’s not secure, well here is an opportunity to practise it without detracting too much from the main objective.

Another benefit, is that it might be easier for students to plot points by counting squares rather than draw accurate, extended ray lines, as pointed out by Mr Blachford on Twitter.

Once students have done a few examples, we can draw attention to some things, for example:

  • If the scale factor is >1, it gets bigger, if it’s <1 it gets smaller
  • The scale factor applies to each side length of the shape (but not the area…)
  • Negative scale factors always place the image the “other side of the CoL”
  • All ray lines must go through the CoL. This is how it is constructed.

I would use Geogebra (as in fact I did to create these images) to examine what happens when we move the CoL. (Note: Geogebra uses the US terminology Dilation rather than Enlargement – which actually is more descriptive of what we are doing, isn’t it?!)

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And from there we can go onto questions that show the enlargement and ask for a description.  I would use Geogebra for that bit as well as it is very easy to create images that can then be used to ask learners to describe what they see.

Just for fun, I had a go at a 3D version in Geogebra.  I’l leave you to decide whether or not it adds anything. You should be able to access it here. This is what it looks like:

3D dilation.gif

This is an example.  It’s one way of doing it. For other topics, for example fractions, I would prefer to spend a decent chunk of time on building conceptual understanding before focussing on algorithms to get right answers. But that’s for another post…

 

 

 

 

Visualising Enlargements

I had always used Geogebra for this in the past, but now I’ve found this site, I don’t think I’ll bother with that any more.  This has all the settings and options you need. You could get students up to the board to draw on where they think the enlargement will be prior to revealing it.  Or show the enlargement but not the ray lines and they have to work out where the centre of enlargement is.  There are also lots of nice examples to do on worksheets on the site. (click the home button)Screen Shot 2015-11-02 at 21.15.33

One of the many good resources on the NGfL Cymru site.  Shame they don’t exist as an organisation any more but luckily their resources live on!

Extending Transformations

All four transformations (Translation, Reflection, Rotation, Enlargement) with Year 10. It feels like one of those topics that we cover in Year 8, they mostly grasp some of the skills but then forget the details (e.g. to specify the centre of rotation). That’s OK, I reckon. But it does imply that the time in Year 8 is better spent mastering the more tricky skills like reflecting in 45 degree lines than answering GCSE style questions, e.g doing translations using vectors.  These sheets are an old favourite which are great for practising those skills.

I was a bit surprised how long my middle set took to get to grips with it. Inevitably there were some who finished quickly because they found it easier to visualise than others. I was a bit stumped for how to extend it. A quick conversation in the maths office gave me the idea of multiple translations. Of course! Do, say, a reflection followed by a rotation. And then what is the single transformation that has the same effect?

I created a simple extension question:

The shape ABCD has coordinates (-1,1) (-3,1)
(-3,4) and (-1,5) respectively. It is reflected in the line y = x to produce shape A’B’C’D’ and then rotated 900 anticlockwise about (2,2) to create A’’B’’C’’D’’.

What are the coordinates of each of the points A’’, B’’, C’’, D’’ after these TWO transformations?

Describe fully the single transformation which would map ABCD onto A’’B’’C’’D’’.

This document can easily be printed and chopped. I used this geogebra file to create it.Screen Shot 2015-03-25 at 18.47.35

It’s easy to create more questions like this by moving the points of the original shape and changing the transformations in the Geogebra file.