Why is it a 'challange questions'?
\(\cos(2\theta) = 2\cos(\theta)\\ cos^2\theta-\sin^2\theta = 2\cos(\theta)\\ cos^2\theta-(1-\cos^2\theta) = 2\cos(\theta)\\ 2cos^2\theta-1 = 2\cos\theta\\ 2cos^2\theta-2\cos\theta-1 = 0\\\)
Now let x=cos (theta) and solve for x.
etc
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