14. The three dimensional non-zero vector \(\boldsymbol { u }\) has the following properties:
- The angle \(\theta\) between \(\boldsymbol { u }\) and the vector \(\left( \begin{array} { l } 1
5
9 \end{array} \right)\) is acute. - The (non-reflex) angle between \(\boldsymbol { u }\) and the vector \(\left( \begin{array} { l } 9
5
1 \end{array} \right)\) is \(2 \theta\). - \(\boldsymbol { u }\) is perpendicular to the vector \(\left( \begin{array} { l } 1
1
1 \end{array} \right)\).
Find the angle \(\theta\).
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