\begin{exercise}{M3}{Invertible matrices}{0002}
\begin{exerciseStatement}
Explain why the matrix \(N= \left[\begin{array}{cccc}
1 & -2 & 8 & 7 \\
0 & 1 & -2 & -1 \\
1 & -1 & 6 & 7 \\
1 & 0 & 4 & 5
\end{array}\right] \) is or is not invertible.
\end{exerciseStatement}
\begin{exerciseAnswer}
\[\operatorname{RREF} \left[\begin{array}{cccc}
1 & -2 & 8 & 7 \\
0 & 1 & -2 & -1 \\
1 & -1 & 6 & 7 \\
1 & 0 & 4 & 5
\end{array}\right] = \left[\begin{array}{cccc}
1 & 0 & 4 & 0 \\
0 & 1 & -2 & 0 \\
0 & 0 & 0 & 1 \\
0 & 0 & 0 & 0
\end{array}\right] \]
\(N\) is not invertible.\end{exerciseAnswer}
\end{exercise}
