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Reverse percentages and compound interest

November 15, 2017 1 comment

The other day a discussion arose in my year 10 class that I found rather interesting. There was a question on interest which incorporated compound interest and reverse percentages. One student was telling the other how to find the answer to the reverse part, “you need to divide it, because it was that amount times by the multiplier to get this amount and divide is the inverse of times.” All good so far, then they discussed how to complete it if it was a reverse of more than one year, “so in that case it’s the new amount dived by the multiplier to the power of how many years.” I was pleased at the discussion so I didn’t really interject.

Then one of them aid, “if I’m looking for two years ago, can’t I just times it by the multiplier to the power -2? Wouldn’t that work.” I thought this was an excellent thought process. The other student disagreed though, sating “no, it has to be divide.” So I thought at this point I’d better interject a little.

“Does it give you the same answer?” I asked. They both thought about it and tried it and discussed it and said yes. So I asked “does it ALWAYS give the same number?” they tried a number of scenarios using different amounts, different interest rates and different numbers of years. Eventually they had convinced themselves. “Yes, yes it is always the same.”

“So is it a valid method then?” I probed. Some more discussion, then one ventured “yes. It must be.”

“Why does it work?”, I then asked. And left them discussing it.

When I came back to the pair I asked if they could explain why it works and one of them said, “we think that it’s because multiplying by a negative power is the same as dividing by the positive version.”

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Oblongs

November 10, 2017 5 comments

Last week while we were waiting for a swimming lesson to start my daughter told me that one of her teachers had got “higgledy piggledy” about oblongs. I asked what she meant and she said that she’d accidentally called one a rectangle and had to correct herself and had informed the class that at her last school she’d had to call them rectangles but at this school had to call them oblongs and sometimes got higgledy piggledy about this. I asked my daughter why they couldn’t call them rectangles and she said that it was because squares can be rectangles too.

This set off a lengthy chain of thoughts in my head. Firstly, I was quite impressed by the fact a 5 year old could articulate all this about knowledge about shapes so well. Then I thought, does it really matter whether they call them oblongs or rectangles? Then I thought, wait a minute, why are we prohibiting the use of rectangle because it can also mean a square, but we are not prohibiting the use of oblong when it can also mean an ellipse? My chain of thought then jumped down a rabbit hole questioning whether we should actually be referring to regular or equilateral rectangular parallelograms and non – regular/equilateral parallelograms. Why are we allowing children to call a shape a triangle, when it is one possible type of triangle in a family of triangles, but not allowing them to call a shape a rectangle when it is only one possible rectangle in a family of rectangles. These thoughts stewed around in my head for a while and I thought I’d ask the twittersphere for their opinions on the matter.

These opinions fell into a couple of camps. The first cam thought that oblong was a nice enough word and they didn’t mind others using it but preferred not to themselves. The second camp felt that it was important to distinguish between an oblong and a square so important to use oblong not rectangle and the third camp thought that actually it was better to use rectangles due to the elliptical oblongs. I questioned some of the respondents from the second two groups a little further to see why they fell into these groups. Those in the second seemed unaware that the word oblong also meant ellipse and those in the third thought it was more important to excluded ellipses than squares. Stating that it was easy enough to explain away the special case that is the square.

I’ve spend rather a lot of time considering this, and am now not really sure what I think on the issue. I can’t see a problem with using a rectangle and explaining away the square as a special case. We call all triangles triangles and expand as and when required. No one bothers about calling a non-rectangular parallelogram a parallelogram, despite the fact that that could mean a rectangle. But again I’m not sure I’m massively strongly against the term oblong either. It could open up a good discussion about the term and how it could apply to ellipses, although this probably is a little too much for a year 1 classroom. I think I’m leaning towards rectangle as a preference though, as explaining away a special case is, for me, much more preferable than ignoring a whole class of oblongs.

 

If you have views on this, whichever way you lean, I’d love to hear them, either in the comments or via social media.

Categories: #MTBoS, KS2, KS3, Maths, SSM, Teaching Tags: , ,

Dodgy Microsoft Graphics 

October 10, 2017 1 comment

So my new laptop arrived today and I quickly set about using it. It’s a Windows 10 laptop and as such has all the usual Microsoft stuff preloaded in it. I was going to set chrome as the default browser when it suggested I try Microsoft edge as it’s apparently faster and made for Windows 10. When I opened it it showed me this graphic:

Immediately I called shinanegans. The 5% difference between the green and the blue looked far too big. Initially I thought it was just down to the scale starting from 25000 and the size, but looking deeper there are also 4 extra sets if 5 notches on the blue which further add a to the illusion.

All in all a terrible diagram. Poor form Microsoft. Poor form.

An excellent puzzle – alternate methods

July 19, 2017 2 comments

Yesterday I wrote this post looking at a nice puzzle I’d seen and how I solved it.

The puzzle again:

Lovely, isn’t It?

After I published my previous post I wondered if I may have been better using a vector approach or a coordinate geometry approach. So I gave them a try.

Coordinate Geometry

I started by sketching the figure against an axis.

I place the origin at the centre of the circle, worked out the equation if the circle and the right leg of the triangle and solved simultaneously for x. Giving x =1 and x=1/3. These x values correspond to half the base of each triangle, which shows the scale factor from the large triangle to the small one is 1/3. As the area of the large one is rt2 this gives the area of the small as rt2/9.

I like this method, probably a little better than the one prior to it.

Vectors

First I sketched it out and reasoned I could work it out easy enough with 4 vectors.

I saw that I could write AC as a sum of two others:

I knew that the length of AC was 1 so I used Pythagoras’s Theorem to calculate mu. It left me with the exact same quadratic to solve. This time mu was the fraction of DB needed so was automatically the length scale factor. The rest falls out as it did before.

As well as this, Colin Beveridge (@icecolbeveridge), maths god and general legend, tweeted a couple of 1 tweet solutions. First he used trig identities:

Trig Identities 

I assumed this was right, but checked it through to ensure I knew why was going on:

We can see beta is 2 x alpha and as such the tan value is correct. The cos value (although it is missing a negative sign that I’m sure Colin missed to test me) follows from Pythagoras’s Theorem:

This is again the scale factor as it is half the base of the small triangle and the base if large triangle is 2.

Complex Numbers

Then Colin tweeted this:

At first I wasn’t totally sure I followed so I asked for further clarification:


I had a moment of stupidity:

And then saw where Colin was going. I tried to work it through, by way of explaining here in a better manner.

I sketched it out and reasoned the direction of lines:

Then I normalised that and equated imaginary parts to get the same scale factor:

I am happy that is is valid, and that it shows Colin is right, but I’m not entirely sure this as the exact method Colin was meaning. He has promised a blog on the subject so I will add a link when it comes.

I like all these methods. I dontvthink I would have though of Colin’s methods myself though. I’d love to hear another methods you see.

An excellent puzzle

July 18, 2017 3 comments

Today I saw this tweet:

The puzzle looked grand. Thanks to those people that tweeted at me to make sure if seen it, it’s much appreciated.

The puzzle itself is:

I drew it out and labelled a few things:

But soon realised that it’s impossible unless you make assumptions. 

With the assumption that the vertex of the triangle is at the midpoint if the line I was in a position to have a good crack at it. My first thought, as is often the case, was to run at it using right angled triangles:

My initial thought was to use right angled trigonometry, but I realised I’d probably need to approximate or use some maths software and that would take a bit of the fun out of it. I presumed I’d be able to find an exact answer in a better way.

I realised the big triangle and the green triangle were similar and I could easily work out the area of the big triangle.

I then realised I didn’t have the scale factor. I went back to rats.

Then realised I had another similar isoceles triangle to play with:

Using similarity I found the “base” of DFG and used that to find length EG. Thus giving me a scale factor between the blue and the green triangle.

As mentioned previously I knew that rt2/(SF^2) was the green area so using the scale factor of 3 I got the required area to be rt2/9. 

I would like to say that’s what I did. That’s what I see I should have done while writing this up. But it’s not what I did at the time. I took a longer way round. I got giddy with triangles:

Used  Pythagoras’s Theorem to find the peep height and found the area that way.

Luckily I got the same answer.

I then saw the same tweeter had tweeted this:

This is the same question but altered slightly in the information given and what is required as the final solution. If you make the same assumption it follows from the tan ratio that all the distances are the same, so you need to do the same to that point and then find the ratio green area / blue area. I’d done most of it above, so I finished it off:

I love this puzzle, and I hope to use it in my classes next year. I may give it to year 12 tomorrow and see if they can crack it. I think I prefer the second variation. I’d love to hear your thoughts on it, and how you solved it. Let me know in the comments or via social media. 

Thoughts on the understanding paradox and introducing trigonometry

July 14, 2017 4 comments

Recently I read a blog entitled “The understanding paradox” (William, 2017) which discussed the idea of maths teaching and put forward the idea that actually, it is better to bypass understanding when first teaching a topic and then fill that understanding in later. This was then applied to the teaching of right angled triangle trigonometry in an example that I found confusing to say the least.

The author, Rufus William, suggested that when teaching trig for the first time we should be solely teaching procedurally using SOHCAHTOA as a mnemonic, but then went on to say we shouldn’t be discussing ratio or similarity and how that links until later on. This confused me as the mnemonic SOHCAHTOA is designed to help you remember the trig ratios. I.e. Sine is the ratio of the opposite side over the hypotenuse. Just by teaching that you ARE teaching the trig ratios and purely by the fact that you are teaching the students that this will work for all right angled triangles you are telling the students that the ratios are the same for any triangle with the same angle no matter what the length of the sides are. THIS IS THE VERY DEFINITION OF SIMILAR TRIANGLES.

This perplexed me a lot and I spent a lot of time thinking about it and asking the author to elaborate on what he meant. The only way I can fathom to teach this without reference to ratio and similarity would be to say: ” “SOHCAHTOA” it gives you 3 triangles. Label the sides circle them to see which triangle you use. Put numbers in, cover the missing one, its either a divide or a times”. To me this seems like a backwards way to go about things. It feels like you are teaching them unnecessary procedures to avoid discussing the underlying concepts of trigonometry, and it doesn’t really make sense to me.

I find that by the time students reach right angled triangle trigonometry they have already met the concept of similarity, I like to use this a way in to discussing the topic and to show that ratio of two sides that are the same in relation to an angle will be the same for all similar triangles. Students will have always encountered simplifying fractions before they meet trig and as such can see why this is. This is when I specifically discuss the sine, cosine and tangent ratios and introduce the procedural manner in which they can solve the problems, although I do avoid the dreaded formula triangles (for many reasons which I have blogged about here). I will show them some common mnemonics, and SOHCAHTOA is one of them. I’m not a fan of mnemonics personally, I’ve never found them that useful except for musical ones, but I know a lot of people do.

Rufus does make some salient points in his post about teachers who refuse to allow students to memorise things and the dangers this will have on learning. Although I’m not entirely sure that they exist, and if they do I certainly don’t think there are many of them. I’ve certainly never met any.

He also suggests that students cannot have a full understanding of the ins and outs of trigonometry when they first meet it. I would very much agree with him in that respect, I know many people who have taught trigonometry for decades and still don’t, but I don’t think that means we have to bypass all information.

Reference List:

Cavadino, S.R. 2014. Formula Triangles. 12th October. Cavmaths. [online] accessed 14th July 2017. available: https://cavmaths.wordpress.com/2014/10/12/formula-triangles/

Cavadino, S.R. 2016. Catchy Mnemonics. 16th September. Cavmaths. [Online] accessed 14th July. Available: https://cavmaths.wordpress.com/2016/09/16/catchy-mnemonics/

William, R. 2017. The understanding paradox. 7th July. No easy answers. [online] accessed 14th July 2017. available: https://noeasyanswerseducation.wordpress.com/2017/07/07/the-understanding-paradox/

 

Group Work Issues

July 14, 2017 3 comments

Recently I wrote this post (2017) that highlights various ways that I can see group work being of benefit to students study in mathematics.  In the post I allude to there being many issues around group work that can have a detrimental effect on the learning of the students and I intend to explore them a little further here.

The benefits of group work can be vast, and are often tied to the discussion around the mathematics involved in a way consistent with the writings of Hodgen and Marshall (2005), Mortimer and Scott (2003), Piaget (1970), Simmons (1993), Skemp (1987) and Vygotsky (1962) amongst others. These perceived benefits give the students a chance to try things, make mistakes, bounce ideas around and then find their way through together. Seeing the links between the things they know and its application within new contexts or the links between different areas of maths.

So what are the down sides?

Good et al. (1992) warn that group work can reinforce and perpetuate misconceptions. This is an idea that is also expressed by von Duyke and Matsov (2015) who feel that the teacher should be able to step in and correct any misconceptions that the students express, although this would be difficult in a classroom where a number of groups are working simultaneously and it also goes against the feelings expressed by some researchers, such as Pearcy (2015), that students should be allowed to get stuck and not receive hints. This is a tricky one to balance. As teachers we clearly do not want misconceptions becoming embedded within the minds of our students, but we do want to allow them time to struggle and to really get to grips with the maths. I try to circulate and address misconceptions when they arise but in a manner that allows students to see why they are wrong, but not give them the correct answer.

Another potential pitfall of group work is related to student confidence. Some students worry about being wrong and as such will not speak up. This is an issue that transcends group work and that we need to be aware of in all our lessons and is discussed at length in “inside the black box” (Black and Wiliam, 1998). It is part of our jobs as teachers to create an environment where students do not fear this, and are comfortable with talking without fear of being laughed at. I try to create a culture where students know it’s better to try and be wrong than not to try at all. This classroom culture is discussed by Hattie (2002) as an “optimal classroom climate” and it is certainly a good aim for all classrooms.

The other main downside to group work is behaviour related (Good et al., 1992). Group work can be more difficult to police, and it can become difficult to check that everyone is involved if you have a large class that is split into many groups. This can give rise to the phenomenon known as “Social Loafing”, which is where some members of the group will opt out in order to have an easy ride as they feel other group members will take on their work as well (Karau and Williams, 1993). This is something that teachers need to consider and be wary of. The risk of these issues having a negative impact on learning can vary wildly from class to class and from teacher to teacher. I would advise that any teacher who is considering group work needs to seriously consider the potential for poor behaviour and social loafing to negatively impact the lesson and to think about how they ensure it doesn’t. Different things work for different people. Some people assign roles etc. to groups. Some set up a structure where students can “buy” help from the teacher or other groups. Often a competitive element is introduced. All of these can be effect or not, again depending on the class and on the teacher so it is something we need to work on individually. I’ve written before about one method I’ve had some success with here (2013).

So there are some of the worries around group work and thoughts on what needs to be considered when embarking on it. As mentioned in my previous post, I feel that group work is an inefficient way to introduce new concepts and new learning, but I do see it as something that can be very effective when building problems solving skills and looking at linking areas of mathematics together.

What are your thoughts on group work? And what are your thoughts on the issues mentioned in the article? I’d love to hear them via the comments or on social media.

Reference list / Further reading:

Black, P. and Wiliam, D. 1998. Inside the black box: Raising standards through classroom assessment. London: School of Education, King’s College London.

Cavadino, S.R. 2013. Effective Group Work. 5th July. Cavmaths. [online] accessed 14th July 2017. Available: https://cavmaths.wordpress.com/2013/07/05/effective-group-work/

Cavadino, S.R. 2017. Student led learning in maths. 13th July. Cavmaths. [online] accessed 14th July 2017. Available: https://cavmaths.wordpress.com/2017/07/13/student-led-learning-in-maths/

Good, T.L., McCaslin, M. and Reys, B.J. 1993. Investigating work groups to promote problem-solving in mathematics. In: Brophy, J. ed. Advances in research on teaching: Planning and managing learning tasks and activities. United Kingdom: JAI Press.

Hattie, J. 2012. Visible learning for teachers: Maximizing impact on learning. Abingdon: Routledge.

Hodgen, J. and Marshall, B. 2005. Assessment for learning in English and mathematics: A comparison. Curriculum Journal. 16(2), pp.153–176.

Karau, S.J. and Williams, K.D. 1993. Social loafing: A meta-analytic review and theoretical integration. Journal of Personality and Social Psychology. 65(4), pp.681–706.

Mortimer, E. and Scott, P. 2003. Meaning making in secondary science classrooms. Maidenhead: Open University Press.

Pearcy, D. 2015. Reflections on patient problem solving. Mathematics Teaching. 247, pp.39–40.

Piaget, J. 1970. Genetic epistemology. 2nd ed. New York: New York, Columbia University Press, 1970.

Simmons, M. 1993. The effective teaching of mathematics. Harlow: Longman.

Skemp, R.R. 1987. The psychology of learning mathematics. United States: Lawrence Erlbaum Associates.

von Duyke, K. and Matusov, E. 2015. Flowery math: A case for heterodiscoursia in mathematics problems solving in recognition of students’ authorial agency. Pedagogies: An International Journal. 11(1), pp.1–21.

Vygotsky, L.S. 1962. Thought and language. Cambridge, MA: M.I.T. Press, Massachusetts Institute of Technology.

 

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