trigonometry

Question: If $\sin \beta $is the geometric mean between $\sin \alpha $and $\cos \alpha ,$then $\cos 2\beta $is equal to

1) $2{{\sin }^{2}}\left( \frac{\pi }{4}-\alpha \right)$

2) $2{{\cos }^{2}}\left( \frac{\pi }{4}-\alpha \right)$

3) $2{{\cos }^{2}}\left( \frac{\pi }{4}+\alpha \right)$

4) $2{{\sin }^{2}}\left( \frac{\pi }{4}+\alpha \right)$

Solution:

Explanation: No Solution

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