Combined transformations inquiry
Mathematical inquiry processes: Generate examples; test particular cases; analyse structure. Conceptual field of inquiry: Reflections, rotations and translations; combined transformations.
The statement in the prompt is always true. If the lines are perpendicular, then the two reflections can be represented by a 180o rotation about the point at which the lines intersect. If they are parallel, the two reflections can be represented by a translation. Students' initial questions and observations have included:
What do 'map', 'object', and 'image' mean?
How many (and what) transformations are there?
How would you arrange the lines of symmetry?
Can you do more than two reflections?
Could we use any shape?
You can only get back to the object by doing two 'reverse' reflections.
Could the shape be three-dimensional?
Does it work for any shape?
Should the shape have a line of symmetry?
In the orientation phase, students often attempt to draw a diagram to illustrate the prompt.
After a discussion about how the two lines are arranged, students might explore reflections of their own shapes using the templates below. Depending on prior learning, the teacher could decide (or respond to students' requests) to explain how the combined reflections can be represented by one transformation. This leads naturally into the concepts of rotation (for reflections in perpendicular lines) and translation (for reflections in parallel lines).
A second stage of the inquiry starts when students suggest combining different pairs of the three transformations. Questions that have arisen at this point include:
Does the order of the transformations matter?
Will you be able to use the same transformation for a 90o or 270o rotation combined with a translation or reflection as you do for a rotation of 180o?
How is the result different if the shape has one or more lines of reflection? (Shall we exclude shapes with lines of symmetry?)
Is it possible to do the combined transformation in more than one way?
Groups of students or the whole class might generate a list of the possible combinations and then divide up the list to explore. The findings can feed into a class record of the inquiry.
Students have extended the inquiry by looking at three transformations (see illustration below). Including an enlargement as one of the transformations also provides greater challenge. The class might decide if it is possible to represent an enlargement with a positive scale factor and a rotation by a single enlargement using a negative scale factor.
Guided inquiry in year 8
Clare Gribben used the prompt with her year 8 class at Bedford Girls' School (Bedford, UK). It was the students' first inquiry and Clare reports that "they struggled at the start and needed lots of guidance." The pictures above show the initial responses of two students. In the top picture, the student has impressively distinguished between (and colour coded) three types of responses: questions, ideas and 'diagrams of what I think we might be doing'. Clare comments, "It was interesting reading the prompt sheets after the lesson as not everything came up in the discussion."
Classroom inquiry in year 8
These are the initial observations and questions from two mixed-attainment year 8 classes:
Many students went on to explore other combined transformations. As they inquired, students decided whether they needed instruction and structured practice in order to carry out reflections, translations and rotations confidently. The sheets show some of the results of the exploration phase.
Discussions of the prompt
Particular to general
Don Steward discussed the combined transformation prompt in his presentation at the joint Maths Hubs conference at Villa Park in June 2017. In his presentation, Don explained the process he follows for designing mathematical tasks. Starting with an exam question, which focuses on a particular case, he explores how it could be generalised. By following the same process in the classroom, students appreciate the general mathematical structure underlying a particular question. Don showed how the reasoning that was required to solve a question on an Edexcel GCSE paper (paper 2, June 2017) can be developed through the combined transformation inquiry. You can see the section of Don’s presentation related to the inquiry here. It starts with the examination question and then considers initial pathways the inquiry might take.
Paul Aniceto and Kent Nobes (grade 5 teachers from Ontario, Canada) co-planned an inquiry on translation and reflection and then started a discussion on twitter. They posed this question to the pupils as a stimulus to inquiry: If you translate a symmetric trapezoid in any direction, then can you use a different transformation to get to that spot? An alternative prompt would be to present this as a statement, which might lead to students generating their own questions and conjecturing about the truth of the statement: If you translate a symmetric trapezoid in any direction, then you can use a different transformation to get to that spot.
Transformation 1 followed by transformation 2, gives the same image as transformation 2 followed by transformation 1.
Dan Pearcy (the Head of Mathematics at The International School of Lausanne) used this prompt at end of a unit on transformations. In being more general than the combined transformations prompt, it assumes knowledge of more transformations. The original prompt is accessible to students who have come across only reflections. The teacher can introduce new knowledge about rotations and translations in order to confirm the prompt is true for specific cases of combined reflections.
Any composition of two transformations can be replaced by a single one.
Daniela Vasile - a Head of Mathematics in Hong Kong - suggested another prompt: Any composition of two transformations can be replaced by a single one. This suggestion is very interesting because it proposes a conjecture about the general case. Inquiry Maths prompts tend to isolate a particular case for two reasons. Firstly, students can access the prompt without being overloaded by many different possibilities. Secondly, the inductive movement from examples to a generalisation can be carried out by the students themselves. However, a general statement could be appropriate to challenge older students or experienced inquirers.