If `tan theta tan phi=sqrt((a-b)/(a+b))`, prove that
`a-bcos 2theta)(a-b cos 2phi)` is independent of `theta` and `phi`.

Explanation

To prove that \( (a - b \cos 2\theta)(a - b \cos 2\phi) \) is independent of \( \theta \) and \( \phi \) given that \( \tan \theta \tan \phi = \sqrt{\frac{a - b}{a + b}} \), we will follow these steps:

### Step 1: Express \( \cos 2\theta \) and \( \cos 2\phi \) in terms of \( \tan \theta \) and \( \tan \phi \)

Using the double angle formula for cosine, we have:
\[ \cos 2\theta = \frac{1 - \tan^2 \theta}{1 + \tan^2 \theta} \]
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