Solution CRS.C31 Robert Beezer
Theorem BCS is the right tool for this problem. Row-reduce this matrix, identify the pivot columns and then grab the columns of $A$ with the same indices for the set $T$. The matrix $A$ row-reduces to \begin{equation*} \begin{bmatrix} \leading{1} & 0 & 3 & 0 & 0 \\ 0 & \leading{1} & -2 & 0 & 0 \\ 0 & 0 & 0 & \leading{1} & 0 \\ 0 & 0 & 0 & 0 & \leading{1} \end{bmatrix} \end{equation*} So $D=\set{1,\,2,\,4,\,5}$ and then \begin{equation*} T=\set{\vect{A}_1,\,\vect{A}_2,\,\vect{A}_4,\,\vect{A}_5}= \set{ \colvector{2 \\ 1 \\ -1 \\ 2},\, \colvector{1 \\ -1 \\ 2 \\ -1},\, \colvector{-1 \\ 1 \\ 0 \\ -1},\, \colvector{2 \\ 1 \\ 1 \\ 2} } \end{equation*} has the requested properties.