a
(a) \(\cos \theta = \frac{{\overrightarrow A .\overrightarrow B }}{{|A||B|}} = \frac{{(3\hat i + 4\hat j + 5\hat k)\,(3\hat i + 4\hat j - 5\hat k)}}{{\sqrt {9 + 16 + 25} \sqrt {9 + 16 + 25} }}\)
\( = \frac{{9 + 16 - 25}}{{50}} = 0\)
\(⇒\) \(\cos \theta = 0\),
\(\therefore \) \(\theta = 90^\circ \)