Question #258889

W=(x,y,z)R33x2y+4z=0.W = {(x, y, z) ∈R3|3x −2y + 4z = 0}.

1
Expert's answer
2021-11-01T18:33:59-0400
3x2y+4z=03x−2y+4z=0

Any vector w=[xyz]\vec w=\begin{bmatrix} x \\ y \\ z \end{bmatrix} in WW satisfies 3x2y+4z=03x-2y+4z=0 or equivalently x=23y43z.x=\dfrac{2}{3}y-\dfrac{4}{3}z.

Thus we have

w=[xyz]=[23y43zyz]=y[2/310]+z[4/301]\vec w=\begin{bmatrix} x \\ y \\ z \end{bmatrix}=\begin{bmatrix} \dfrac{2}{3}y-\dfrac{4}{3}z \\ y \\ z \end{bmatrix}=y\begin{bmatrix} 2/3 \\ 1 \\ 0 \end{bmatrix}+z\begin{bmatrix} -4/3 \\ 0 \\ 1 \end{bmatrix}

Let

u1=[2/310],u2=[4/301]\vec u_1=\begin{bmatrix} 2/3 \\ 1 \\ 0 \end{bmatrix}, \vec u_2=\begin{bmatrix} -4/3 \\ 0\\ 1 \end{bmatrix}

The above computation shows that any vector w\vec w  in WW can be written as a linear combination of the vectors u1,u2.\vec u_1, \vec u_2.

Hence the set {u1,u2}\{\vec u_1, \vec u_2\} is a spanning set for the subspace W.W.

We claim that {u1,u2}\{\vec u_1, \vec u_2\} is a linearly independent set.

Consider


a1u1+a2u2=0a_1\vec u_1+a_2\vec u_2=\vec 0

The equation can be written as


[(2/3)a1(4/3)a2a1a2]=[000]\begin{bmatrix} (2/3)a_1-(4/3)a_2 \\ a_1 \\ a_2 \end{bmatrix}=\begin{bmatrix} 0 \\ 0 \\ 0 \end{bmatrix}

Comparing entries, we obtain a1=a2=0.a_1=a_2=0.

Thus the equation a1u1+a2u2=0a_1\vec u_1+a_2\vec u_2=\vec 0 has only the zero solution and hence the vectors u1,u2\vec u_1, \vec u_2

are linearly independent.

We found a basis


{u1=[2/310],u2=[4/301]}.\bigg\{\vec u_1=\begin{bmatrix} 2/3 \\ 1 \\ 0 \end{bmatrix}, \vec u_2=\begin{bmatrix} -4/3 \\ 0\\ 1 \end{bmatrix}\bigg\}.



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