Chapter 1: Problem 13
The set of solutions to the system of linear equations $$ \begin{array}{r} x_{1}-2 x_{2}+x_{3}=0 \\ 2 x_{1}-3 x_{2}+x_{3}=0 \end{array} $$ is a subspace of \(R^{3}\). Find a basis for this subspace.
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Chapter 1: Problem 13
The set of solutions to the system of linear equations $$ \begin{array}{r} x_{1}-2 x_{2}+x_{3}=0 \\ 2 x_{1}-3 x_{2}+x_{3}=0 \end{array} $$ is a subspace of \(R^{3}\). Find a basis for this subspace.
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Prove the following properties of the cross product: (a) \(u \times v=-(v \times u)\) (d) \(u \times(v+w)=(u \times v)+(u \times w)\) (b) \(u \times u=0\) for any vector \(u\) (e) \((v+w) \times u=(v \times u)+(w \times u)\) (c) \((k u) \times v=k(u \times v)=u \times(k v)\) \((\mathrm{f}) d(u \times v) \times w=(u \cdot w) v-(v \cdot w) u\)
Let \(W_{1}\) and \(W_{2}\) be subspaces of a vector space \(V\). Prove that \(V\) is the direct sum of \(W_{1}\) and \(W_{2}\) if and only if each vector in \(V\) can be uniquely written as \(x_{1}+x_{2}\), where \(x_{1} \in \mathrm{W}_{1}\) and $x_{2} \in \mathrm{W}_{2}$.
Label the following statements as true or false. (a) If \(V\) is a vector space and \(W\) is a subset of \(V\) that is a vector space, then \(W\) is a subspace of \(V\). (b) The empty set is a subspace of every vector space. (c) If \(V\) is a vector space other than the zero vector space, then \(V\) contains a subspace \(W\) such that \(W \neq V\). (d) The intersection of any two subsets of \(V\) is a subspace of \(V\). (e) An \(n \times n\) diagonal matrix can never have more than \(n\) nonzero entries. (f) The trace of a square matrix is the product of its diagonal entries. (g) Let \(\mathrm{W}\) be the \(x y\)-plane in \(\mathrm{R}^{3}\); that is, $\mathrm{W}=\left\\{\left(a_{1}, a_{2}, 0\right): a_{1}, a_{2} \in R\right\\}\(. Then \)W=R^{2}$.
In each part, determine whether the given vector is in the span of \(S\). (a) \((2,-1,1), \quad S=\\{(1,0,2),(-1,1,1)\\}\) (b) \((-1,2,1), \quad S=\\{(1,0,2),(-1,1,1)\\}\) (c) \((-1,1,1,2), \quad S=\\{(1,0,1,-1),(0,1,1,1)\\}\) (d) \((2,-1,1,-3), \quad S=\\{(1,0,1,-1),(0,1,1,1)\\}\) (e) $-x^{3}+2 x^{2}+3 x+3, \quad S=\left\\{x^{3}+x^{2}+x+1, x^{2}+x+1, x+1\right\\}$ (f) $2 x^{3}-x^{2}+x+3, \quad S=\left\\{x^{3}+x^{2}+x+1, x^{2}+x+1, x+1\right\\}$ (g) $\left(\begin{array}{rr}1 & 2 \\ -3 & 4\end{array}\right), \quad S=\left\\{\left(\begin{array}{rr}1 & 0 \\ -1 & 0\end{array}\right),\left(\begin{array}{ll}0 & 1 \\ 0 & 1\end{array}\right),\left(\begin{array}{ll}1 & 1 \\ 0 & 0\end{array}\right)\right\\}$ (h) $\left(\begin{array}{ll}1 & 0 \\ 0 & 1\end{array}\right), \quad S=\left\\{\left(\begin{array}{rr}1 & 0 \\ -1 & 0\end{array}\right),\left(\begin{array}{ll}0 & 1 \\ 0 & 1\end{array}\right),\left(\begin{array}{ll}1 & 1 \\ 0 & 0\end{array}\right)\right\\}$
Prove: For any complex numbers \(z, w \in \mathbf{C},|z+w| \leq|z|+|w|\)
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