Let \(A\) be the matrix with columns the above vectors: \[\begin{bmatrix} 2 & 3 \\ 0 & -3 \\ 0 & 3 \\ 4 & -9 \end{bmatrix}\] Reduced row echelon form (RREF) of \(A^T\): \[\begin{bmatrix} 1 & 0 & 0 & 2 \\ 0 & 1 & -1 & 5 \end{bmatrix}\] Basis for \(W^\perp = (\textrm{Col}A)^\perp = \textrm{Nul} A^T\): \[\begin{bmatrix} 0 \\ 1 \\ 1 \\ 0 \end{bmatrix}, \qquad \begin{bmatrix} -2 \\ -5 \\ 0 \\ 1 \end{bmatrix}.\]

Problem:

Let \(W\) be the subspace spanned by the vectors    
\[
\begin{bmatrix} 
 2  \\ 
 0  \\ 
 0  \\ 
 4
\end{bmatrix}, \qquad 
\begin{bmatrix} 
 3  \\ 
 -3  \\ 
 3  \\ 
 -9
\end{bmatrix}.\]
Find a basis for the orthogonal complement \(W^\perp\) of \(W\).

\begin{solution}
Solution:

Let \(A\) be the matrix with columns the above vectors:
\[\begin{bmatrix} 
2 & 3   \\ 
0 & -3   \\ 
0 & 3   \\ 
4 & -9
\end{bmatrix}\]
Reduced row echelon form (RREF) of \(A^T\):
\[\begin{bmatrix} 
1 & 0 & 0 & 2   \\ 
0 & 1 & -1 & 5
\end{bmatrix}\]
Basis for \(W^\perp = (\textrm{Col}A)^\perp = \textrm{Nul} A^T\):   
\[\begin{bmatrix} 
 0  \\ 
 1  \\ 
 1  \\ 
 0
\end{bmatrix}, \qquad 
\begin{bmatrix} 
 -2  \\ 
 -5  \\ 
 0  \\ 
 1
\end{bmatrix}.\]
\end{solution}