Question #150984
Which of the following statements are true ?
Give reasons for your answers.
(a) If A belongs to Mn(R), then rank (A)=det(A).
(b) There is one and only one unitary matrix in
Mn(R).
(c) If U and V are subspaces of a vector space
W over R, then U U V is also a subspace of W.
(d) Given a linear transformation T from
R4 to R6, rank (T) + nullity (T) = 6.
(e) The relation R, defined on the set of lines in
R² by `L1 R L2 iff L1 and L2 intersect', is an
equivalence relation.
1
Expert's answer
2020-12-17T18:30:16-0500

a) False. The matrix (1001)\begin{pmatrix} 1&0\\0&1 \end{pmatrix} is of rank 2 and determinant 1.

b) False. For example, (1001)\begin{pmatrix} 1&0\\0&1 \end{pmatrix} and (0110)\begin{pmatrix} 0&1\\1&0 \end{pmatrix} are unitary matrices.

c) False. For example, take UU to be the xx-axis and VV yy-axis, both subspaces of R2\mathbb{R}^2. Their union includes both (1,0) and (0,1), whose sum, (1,1), is not in the union. Hence, the union is not a vector space.

d) False. According to Rank-Nullity theorem, rank(T)+nullity(T)=dim(R4)=46.\text{rank}(T)+\text{nullity}(T)=\dim(\mathbb{R^4})=4\neq6.

e) False. The relation RR is not transitive. For example, take L1,  L2 and L3L_1,\;L_2\text{ and } L_3 are the lines of equations y=x,  y=x and y=x1y=x,\;y=-x\text{ and }y=x-1 respectively. We have

L1RL2 and L2RL3, but L1L3.L_1\,R \,L_2\text{ and }L_2\, R\, L_3,\text{ but }L_1\not R\, L_3.


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