4 You are given that \(\mathbf { a } = \left( \begin{array} { c } 1
2
- 1 \end{array} \right)\) and \(\mathbf { b } = \left( \begin{array} { c } 3
- 1
k \end{array} \right)\).
- Find the angle between \(\mathbf { a }\) and \(\mathbf { b }\) when \(k = 2\).
- Find the value of \(k\) such that \(\mathbf { a }\) and \(\mathbf { b }\) are perpendicular.