Course MT3818 Topics in Geometry

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## Exercises 1

1. Prove that the set of rigid motions of the plane R2 forms a group.
Two figures in the plane are called congruent if there is a rigid motion mapping one to the other. Prove that figures congruent to the same figure are congruent to each other.
(If you know what the word means) deduce that congruence is an equivalence relation. 2. In the diagram shown the three triangles A, B and C are congruent. Find a rotation about some point of the plane which maps A to B.
Find one that maps B to C and one that maps A to C.

3. The corners of a tile in the form of an equilateral triangle are labelled A, B, C anticlockwise. Describe the result of rotating the tile anticlockwise by π/3 (= 60°) about A, then by the same amount about B and C ?
What happens if you rotate by this same angle first about A, then about C and finally about B ?

4. If f is a bijection from the plane to itself which preserves all lengths
(i.e. d(x, y) = d(f(x), f(y)) for all x, y)
show that f also preserves all angles.
Give an example of a bijection which preserves all angles but not lengths.

5. Prove that the determinant map is a group homomorphism from the group GL(n, R) of all invertible n × n real matrices to the multiplicative group R - {0}.
Deduce that SL(n, R) is a normal subgroup of GL(n, R).

6. Let A be an n × n matrix. Prove that A is orthogonal (i.e. has orthonormal columns -- of length 1 which are mutually orthogonal) if and only if AtA = I.
Deduce that for such a matrix AAt = I and that A also has orthonormal rows.
Prove this last fact directly for the 2 × 2 matrix by showing that if
a2 + c2 = b2 + d2 = 1 and ab + cd = 0 then ac + bd = 0 and a2 + b2 = c2 + d2 = 1.

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JOC February 2003