If $\cos \theta = \frac{3}{5}$ and $\cos \phi = \frac{4}{5},$ where $\theta $ and $\phi $ are positive acute angles, then $\cos \frac{{\theta - \phi }}{2} = $
If $\cos \theta = \frac{3}{5}$ and $\cos \phi = \frac{4}{5},$ where $\theta $ and $\phi $ are positive acute angles, then $\cos \frac{{\theta - \phi }}{2} = $
- A
$\frac{7}{{\sqrt 2 }}$
- B
$\frac{7}{{5\sqrt 2 }}$
- C
$\frac{7}{{\sqrt 5 }}$
- D
$\frac{7}{{2\sqrt 5 }}$
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- [IIT 1985]
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