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Matrix Meaning

Age 16 to 18
Challenge Level Yellow starYellow star
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Why do this problem?

This problem asks students to consider the geometrical properties of matrix transformations in order to gain a greater understanding of matrix algebra, in 2 and 3 dimensions.
 

Possible approach

The problem works well as a discussion activity. Students could work with a partner and consider each statement first in 2D and then in 3D. After allowing them some time to consider the statements, work with examples, and think about the geometrical interpretation of the situation, bring the class together to discuss their ideas.

Encourage justifications which use geometrical reasoning as well as those using algebra. If a statement is sometimes true, it is important for students to identify when it is true, and geometrically speaking, why there are situations where it is and isn't true.

Encourage the use of sketches to show counterexamples!
 

Key questions

What sorts of rotations and reflections can you have in two dimensions?  What about three dimensions?
When you perform two transformations, does the order matter?
$M$ and $N$ are neither reflections nor rotations - what other types of transformation could they represent?
 

Possible extension

Construct matrices in three dimensions which make each statement true or not true.
Transformations for 10 offers a chance to think about transformations effected by different matrices.
 

Possible support

There are more matrix problems in this feature.

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Follow hints using a little coordinate geometry, plane geometry and trig to see how matrices are used to work on transformations of the plane.

Reflect Again

Follow hints to investigate the matrix which gives a reflection of the plane in the line y=tanx. Show that the combination of two reflections in intersecting lines is a rotation.

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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