Correct option is (a) 90°
\(\vec A.\vec B =(3\hat i + 4\hat j +5\hat k).(3\hat i + 4\hat j- 5\hat k)\)
\(= 3 \times 3 + 4\times 4 + 5\times(-5)\)
\(= 9 + 16 - 25\)
\(= 25 - 25\)
\(=0\)
\(\vec A.\vec B = |\vec A| |\vec B| \cos \theta\)
\(\cos \theta = \frac{\vec A. \vec B}{|\vec A|.|\vec B|}\)
\(= \frac{0}{|\vec A|.|\vec B|}\)
\(= 0\)
\(\cos \theta = 0\)
\(\therefore \theta = 90°\)
