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Resourceaholic: Symmetry

That is the seventh article written within the ‘Dose of Don’ sequence – this one is written by Sudeep Gokarakonda (@boss_maths). Anne Watson posted it on her weblog right here and I’ve replicated it phrase for phrase. For the background on this sequence, please see the put up Traces and Angles on Sq. Grids. My thanks go to Sudeep for giving me permission to share his writing right here.

Dose of Don 7: Symmetry

This can be a contribution to sequence of writings, begun by Anne Watson, which delve into the gathering of duties on Don Steward’s weblog and pull out threads about key concepts in arithmetic that run via a number of of his duties. Direct hyperlinks to all duties talked about are included beneath.

Don was very beneficiant along with his duties and it’s hoped that you’ll return this generosity in the way in which he requested earlier than he died, specifically to donate to

This put up is about symmetry. Point out symmetry, and many individuals’s first ideas may contain symmetry in a firmly geometric context. At college, nearly all college students explicitly encounter reflection and rotational symmetry. For a lot of, their appreciation of symmetry may not lengthen past these concepts.

Symmetry is one thing that permeates arithmetic, and it’s one thing mathematicians can recognise in conditions that aren’t introduced in a typical geometric context. Right here, for instance, is such a scenario:

You could have the next cash totalling 70 pence. In these questions, “quantity” refers to a complete variety of pence.

a) Utilizing solely these cash, what’s the smallest quantity you can not make?

b) Utilizing solely these cash, what’s the largest quantity underneath 70 pence that you simply can’t make?

c) What do you discover about your two solutions?


I keep in mind a lesson involving Pascal’s triangle throughout which I causally talked about its symmetry. One pupil appeared satisfied that I used to be unsuitable to recommend any symmetry right here. On exploring, I realised they had been contemplating the numerals quite than the numbers, so for them:

This barely constrained conception of symmetry was not completely shocking, understanding that the coed’s publicity to the thought had been restricted to the geometric contexts within the GCSE specification.

Listed here are some slides from Don’s duties. Not one of many chosen duties is primarily about symmetry. However, every offers us alternatives to note and discover symmetry.

These are the primary two slides from the duty. Let’s take into account query 16. The frequencies y, w, y, w, y learn the identical forwards and backwards. It’s attainable to show that the imply is at all times 6 with out recognizing this, by establishing and simplifying the next:

However recognizing the symmetry results in an opportunity to generalise. In query 16, if I changed y, w, y, w, y with any symmetric sequence (e.g.) a, b, c, b, a, would the imply nonetheless be 6? If the frequencies had been as an alternative a, b, c, d, a however the imply was nonetheless 6, what might we conclude?

I additionally like tying query 16 again to questions 1 and a pair of utilizing another strategy – appreciating how scores beneath the imply have to be completely “balanced out” by scores above the imply.

For instance, in query 1, provided that the imply is 5,

  • the one rating of three offers a “deficit” of two;
  • the scores of 5 don’t have any impression on the imply;
  • the 2 scores of 6 give a “surplus” of two; and
  • the 4 scores of seven give a “surplus” of 8.
General, I subsequently nonetheless want an additional “deficit” of 8. Since a single rating of 4 ends in a deficit of 1, I want eight such scores, so a = 8.

This strategy is, to me, is mentally much less taxing than establishing and fixing the next in my head:

The “balancing out” strategy in fact works in all questions, nevertheless it’s maybe greatest illustrated utilizing these questions the place the imply is an integer (i.e. questions 1, 2 and 16), as a result of the arithmetic is stored comparatively easy. I really like how this part of the duty could be bookended on this manner.

The road y = x is a line of symmetry on all 4 slides – even the place we solely have the primary quadrant. This line of symmetry might not initially be apparent to all college students. On the very first slide, nevertheless, is the chance to identify that e.g. (1, 12) and (12, 1); (2, 6) and (6, 2) and so forth. are all factors on the hyperbola. Is it essentially the case that if (a, b) is on the curve, then so is (b, a)? What about (–a, –b)?

These questions contain symmetry in a geometrical context, however a chance to contemplate symmetry in a extra refined context pops up on slide 4. I see that 3y + x = 12 is simply y + 3x = 12 with the x and y swapped round. With out even seeing the graphs of those, I sense symmetry in these coefficients. Right here is a chance to ask what occurs, typically, to a graph if I swap x and y round in its equation. College students might make predictions, after which test by making an attempt a number of capabilities utilizing graphing software program. Can they give you an intuitive rationalization for what they observe?

This second could also be a pure one to take a detour to go to (or revisit) self-inverse capabilities—and even pattern one other of Don’s duties, akin to self-inverse and periodic capabilities.

The concept of symmetry crops up when contemplating preparations, possibilities, and plenty of matters in statistics. Don’s duties usually embody stunning but easy visualisations illustrating symmetry:
What I like on this remaining diagram is that, as drawn, it doesn’t have rotational or reflective symmetry. This opens the chance to debate what mathematicians may imply when speaking about symmetry right here. Such discussions ought to hopefully assist broaden the form of conception of symmetry exhibited by my aforementioned pupil of Pascal’s triangle.


As said earlier, not one of the above duties are primarily about symmetry. Don created different duties that I’ve not detailed right here, the place it’s fairly attainable to work via them – and achieve richly – with out recognizing the symmetries in them. However recognizing them offers courses the chance to maybe take a “scenic route” via the duties – one which helps construct up college students’ sense of symmetry in arithmetic.



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