Express \cos(7\theta) in terms of \cos(\theta) only.
\cos^7\theta + 21\cos^5\theta - 35\cos^3\theta + 7\cos\theta
\cos^7\theta - 21\cos^5\theta + 35\cos^3\theta - 7\cos\theta
64\cos^7\theta - 112\cos^5\theta + 56\cos^3\theta - 7\cos\theta
64\cos^7\theta - 80\cos^5\theta + 24\cos^3\theta - \cos\theta