- #1
zeralda21
- 119
- 1
Homework Statement
Find all vectors in $\mathbb R^4$ that are orthogonal to the two vectors
$u_1=(1,2,1,3)$ and $u_2=(2,5,1,4)$.
Homework Equations
Gauss-elimination. Maybe cross-product or Gram Schmidt.
The Attempt at a Solution
a) Denote a vector $u_3=(v_1,v_2,v_3,v_4)$ My desire is to determine $u_3$ so that $\left \langle u_1,u_3 \right \rangle=\left \langle u_2,u_3 \right \rangle=0$
$\left \langle u_1,u_3 \right \rangle=(1,2,1,3)*(v_1,v_2,v_3,v_4)=v_1+2v_2+v_3+3v_4=0$
$\left \langle u_2,u_3 \right \rangle=(2,5,1,4)*(v_1,v_2,v_3,v_4)=2v_1+5v_2+v_3+4v_4=0$
Thus I end up(after Gauss-elimination):
$\begin{pmatrix}
1 &0 &3 &7 \\
0 &1 &-1 &-2
\end{pmatrix}\begin{pmatrix}
v_1\\
v_2\\
v_3\\
v_4
\end{pmatrix}=\begin{pmatrix}
0\\
0
\end{pmatrix}$
which has free variables $v_3,v_4$ but unable to solve.
b) I know that the cross product of two vectors $a$ and $b$ results in a vector orthogonal to $a$ and $b$ that cannot be applied in $\mathbb R^4$. I was also recommended to use Gram-Schmidt but I don't know that yet. Is it more suitable for this problem?