MATH08057 Introduction to Linear Algebra
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MATH08057
Introduction to Linear Algebra
(1) Under what conditions on u and v are the two vectors u + v and u _ v orthogonal? [4 marks]
(2) Let a1 , a2 , a3 be linearly dependent vectors in R3 , which are not all zero. Consider the matrix A with a1 , a2 , a3 as its column vectors.
What are the possible values of rank A? [4 marks]
(3) Let A and B be n × n invertible matrices.
Explain, with reasons, the truth of the following statements.
(a) True or false? For all invertible matrices A and B, (AB)一1 = A一1 B 一1 .
[2 marks]
(b) True or false? For all invertible matrices A and B, (AB)一1 A一1 B 一1 .
[2 marks]
(4) Let S S Rn be a non-empty subspace of Rn .
TRUE or FALSE: Any subset A S S which contains the zero vector must also be a
subspace of Rn . Give a proof or counterexample. [4 marks]
(5) (a) Consider the system of linear equations
3x + 6y + 3X = 9
4x + 8y + 6X = 8
5x + 11y + 8X = 11.
Using Gauss-Jordan elimination, find the solution set of this system. Show which elementary row operation you are using in each step. [5 marks]
(b) Is the system in part (a) equivalent to the system
x + 2y + X = 3
y + 3X = _4
2X = _4?
Explain your reasoning.
(c) Is the system in part (a) equivalent to the system
3x + 6y + 3X = 9
4x + 8y + 6X = 8?
If yes, explain your reasoning. If no, give the solution set of this system.
[2 marks]
(6) (a) Find the intersection between the plane
x + 2y _ z = 3
and the line
x = __06(3).(、) + t __12(1).(、) .
[4 marks] (b) Prove that the distance between the two planes n . x = k1 and n . x = k2 is
lk1 _ k2 l
llnll .
[4 marks]
(c) Find the distance between the planes
2x _ 12y + 10z = 16 and _ x + 6y _ 5z = 3.
[2 marks]
(7) Consider 2 × 2 matrices of the form A = ┌c(a) d(b)┐ . We can think of 2 × 2 matrices as vectors with respect to the standard addition and scalar multiplication operations.
Define the trace of a square matrix as the sum of its diagonal elements. For 2 × 2 matrices the trace is
tr(A) = a + d.
(a) Show the following basic properties of the trace:
(i) tr(A + B) = tr(A) + tr(B), (ii) tr(kA) = k tr(A), (iii) tr(AT ) = tr(A), (iv) tr(AB) = tr(BA)
for A, B any 2 × 2 matrices and k e R a scalar. [6 marks]
(b) Use the properties in (a) to show that the dot product of matrices defined by A ● B = tr(AT B)
satisfies the linearity and symmetry properties of the ordinary dot product of vectors in Rn . That is, show the following identities
(i) A ● (B + C) = A ● B + A ● C ,
(ii) A ● (kB) = k(A ● B),
(iii) A ● B = B ● A
for A, B any 2 × 2 matrices and k e R a scalar. [3 marks]
(c) Consider an orthogonal 2 × 2 matrix P . Show that multiplying any two matrices by an orthogonal matrix leaves their dot product unchanged. That is, verify the following formula
PA ● PB = A ● B
[6 marks]
(8) Let M be the matrix
M = ┌┐
'0 1 1'
(a) Find the eigenvalues of M and the corresponding eigenspaces. [10 marks]
(b) Determine if M is diagonalisable, carefully justifying your answer. If M is
diagonalisable, construct the similarity transformation and resulting diagonal matrix. [5 marks]
(9) Consider the matrix
Tθ = ┌┐
(a) Calculate Tθ T一θ .
(b) Calculate Tθ Tφ .
[2 marks]
If Tθ is invertible, find its inverse. If Tθ is not invertible, justify your reasoning.
[2 marks]
(10) Let A be an n × n orthogonal matrix.
(a) Prove that det A = ·1.
(b) Hence, determine the determinant of B = (det A)An .
[3 marks] [3 marks]
2022-12-01