9. Either give an example of each of the following or explain why it would be impossible.
(a) [2 points] Two orthogonal vectors in R 3 that are linearly dependent.
(b) [2 points] Three orthonormal vectors in R 3 that are linearly dependent.
(c) [2 points] A 3 × 2 matrix Q whose column vectors are orthonormal and QQT 6= I.
(d) [2 points] A 3 × 3 matrix Q whose column vectors are orthonormal and QQT 6= I.
(e) [2 points] Two orthogonal matrices Q1 (3 × 3) and Q2 (3 × 2) such that Q1Q2 is not an orthogonal matrix. [Note: A matrix Q is said to be orthogonal if QTQ = I, i.e., the columns of Q are orthonormal (not orthogonal).]
9. (a). The vectors e1 = (1,0,0)T and e2 = (0,1,0)T are 2 orthogonal vectors in R3 that are linearly dependent.
(b). The vectors e1 = (1,0,0)T , e2 = (0,1,0)T and e3 = (0,0,1)T are 3 orthogonal vectors in R3 that are linearly dependent.
( c). Let Q =
1 |
0 |
0 |
1 |
0 |
0 |
Then QQT =
1 |
0 |
0 |
0 |
1 |
0 |
0 |
0 |
0 |
Thus, the column vectors of Q are orthonormal and QQT ≠ I3.
(d). It is not possible to have a 3x3 matrix Q with orthonormal columns such that QQT ≠ I3 as such a matrix is an orthogonal matrix for which, we necessarily have QQT = I3.
(e). It is not possible to have a 3x3 matrix Q1 and a 3x2 matrix Q2 with orthonormal columns such that Q1Q2 does not have orthonormmal columns. In such a case, the columns of Q1Q2 will be necessarily orthonormal.
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