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QBUS1040
Linear independence Question 5 Consider the following diagram: q1 q˜2 a −βq1 It is known that ∥q1∥ = 1. (a) Write q˜2 in terms of β, a and q1. q˜2 = a− βq1 (b) Given that q1 ⊥ q˜2 i.e. qT1 q˜2 = 0, derive an expression for β in terms of a and q1. 0 = qT1 q˜2 = q T 1 (a− βq1) = qT1 a− β qT1 q1︸︷︷︸ 1 = qT1 a− β → β = qT1 a Question 6 In the Gram-Schmidt algorithm we make the calculation: q˜i = ai − (qT1 ai)q1 − · · · − (qTi−1ai)qi−1 What does this equation look like for: (a) q˜1? q˜1 = a1 (b) q˜2? q˜1 = a2 − (qT1 a2)q1 (c) q˜3? q˜3 = a3 − (qT1 a3)q1 − (qT2 a3)q2 Question 7 Given that qi = q˜i ∥q˜i∥ → qi∥q˜i∥ = q˜i, we can rewrite the equation from the previous question like so: qi∥q˜i∥ = ai − (qT1 ai)q1 − · · · − (qTi−1ai)qi−1 (a) Rearrange the equation to make ai the subject. ai = qi∥q˜i∥+ (qT1 ai)q1 + · · ·+ (qTi−1ai)qi−1 (b) Looking at the equation from part (a), circle the terms which are scalars. ai = qi∥q˜i∥+ (qT1 ai)q1 + · · ·+ (qTi−1ai)qi−1 (c) Re-write the equation from part (a) replacing every scalar with a βi term. ai = βiqi + β1q1 + · · ·+ βi−1qi−1 (d) Is ai a linear combination of q1, . . . qi? Yes. Question 8 Similar to the re-arranging in the previous question, we can also get the formula: qi = (1/∥q˜i∥)(ai − (qT1 ai)q1 − · · · − (qTi−1ai)qi−1) Using this formula: (a) What is q1? q1 = (1/∥q˜1∥)︸ ︷︷ ︸ scalar a1 (b) What is q2 as a linear combination of a1 and a2? (Hint: substitute in your answer from part a) q2 = (1/∥q˜2∥) ( a2 − (qT1 a2)q1 ) = (1/∥q˜2∥)︸ ︷︷ ︸ scalar a2 − (qT1 a2)(1/∥q˜1∥)(1/∥q˜2∥)︸ ︷︷ ︸ scalar a1 Page 1 of 2. QBUS1040 Lecture 5: Question Sheet Solutions Semester 2, 2023 Matrices and matrix examples Question 1 Consider the matrix A = [ 2 6 8 3 −1 0 ] : (a) What is the size of the matrix A? 2× 3 (b) What is A1,2? 6 (c) Is A a tall, wide or square matrix? wide (d) What is the matrix given by A1:2,2:3? [ 6 8 −1 0 ] Question 2 Spot the difference! Below we have written the vector x as both a row vector and a column vector. Circle all of the differences in the notation that you can see. x = (2,− 4,3,8) x = [2 − 4 3 8] Question 3 (a) Given the vectors a1 = (2, 3), a2 = (1, 0) and a2 = (4, 4), what does the matrix A = [a1 a2 a3] look like? a1, a2 and a3 are column vectors. This means that A = [ 2 1 4 3 0 4 ] . (b) Given the vectors a1 = [2 3], a2 = [1 0] and a2 = [4 4], what does the matrix A = [a1 a2 a3] look like? a1, a2 and a3 are row vectors. This means that A = [ 2 3 1 0 4 4 ] . Question 4 (a) What does the matrix 03×2 look like? 0 00 0 0 0 (b) What does the matrix I2 look like? [ 1 0 0 1 ] (c) What does the matrix diag(3,−2) look like? [ 3 0 0 −2 ] Page 2 of 2.