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## 'Nine Eigen' printed from http://nrich.maths.org/

### Why do this problem?

This problem asks a series of questions designed to provoke
students' thinking about matrices which leave vectors fixed, and
the properties that such matrices and vectors would have.

### Possible approach

It may be worthwhile to start with some preliminary work about
matrices in three dimensions. Students could find some examples of
$3 \times 3$ matrices which represent simple rotations and
reflections, which could be used in answering the problem.

The questions divide neatly into three sections - questions
1-3, 4-6 and 7-9. Students could tackle these questions in those
three sections, perhaps working with a partner, and feed back ideas
to the rest of the class after each section is answered.

For each section of questions, ask students to think about what
they are being asked to do, use their intuition to make any initial
comments, then think about the geometry of the situation and
finally use some examples to support their thoughts
algebraically.

### Key questions

What can you say about a rotation that leaves the direction of
a vector unchanged?

What can you say about a reflection that leaves the direction
of a vector unchanged?

### Possible extension

Fix Me or
Crush Me investigates matrices which fix certain
vectors and vectors which are fixed by certain
matrices.

### Possible support

Matrix
Meaning provides some simpler questions about
the possible effects of different types of $3 \times 3$
matrices.