Question : Evaluate the following:$\sqrt{2+\sqrt{2+\sqrt{2+2\cos8\theta}}}$
Option 1: $2 \cos \theta$
Option 2: $2 \cos 2 \theta$
Option 3: $\sin 2 \theta$
Option 4: $\cos 2 \theta$
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Correct Answer: $2 \cos \theta$
Solution :
We know, $\cos2\theta= 2\cos^2\theta-1$
⇒ $\cos2\theta+1= 2\cos^2\theta$
Putting $\theta=4\theta$ on both sides, we get
$\therefore \cos8\theta+1=2\cos^24\theta$
Given, $\sqrt{2+\sqrt{2+\sqrt{2+2\cos8\theta}}}$
= $\sqrt{2+\sqrt{2+\sqrt{2(1+\cos8\theta)}}}$
= $\sqrt{2+\sqrt{2+\sqrt{2(2\cos^24\theta)}}}$
= $\sqrt{2+\sqrt{2+2\cos4\theta}}$
= $\sqrt{2+\sqrt{2(1+\cos4\theta)}}$
= $\sqrt{2+\sqrt{2(2\cos^22\theta))}}$
= $\sqrt{2+2\cos2\theta}$
= $\sqrt{2(1+\cos2\theta)}$
= $\sqrt{2(2\cos^2\theta)}$
= $2 \cos \theta$
Hence, the correct answer is $2 \cos \theta$.
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