8. With respect to a fixed origin \(O\), the lines \(l _ { 1 }\) and \(l _ { 2 }\) are given by the equations $$l _ { 1 } : \mathbf { r } = \left( \begin{array} { r } - 1
5
4 \end{array} \right) + \lambda \left( \begin{array} { r } 2
- 1
5 \end{array} \right) \quad l _ { 2 } : \mathbf { r } = \left( \begin{array} { r } 2
- 2
- 5 \end{array} \right) + \mu \left( \begin{array} { r } 4
- 3
b \end{array} \right)$$ where \(\lambda\) and \(\mu\) are scalar parameters and \(b\) is a constant.
Prove that for all values of \(b \neq 7\), the lines \(l _ { 1 }\) and \(l _ { 2 }\) are skew.