6.
Figure 1
Figure 1 shows a sketch of triangle \(A B C\).
Given that
- \(\overrightarrow { A B } = - 3 \mathbf { i } - 4 \mathbf { j } - 5 \mathbf { k }\)
- \(\overrightarrow { B C } = \mathbf { i } + \mathbf { j } + 4 \mathbf { k }\)
- find \(\overrightarrow { A C }\)
- show that \(\cos A B C = \frac { 9 } { 10 }\)