I'm looking at some textbook question and there is a part where it writes this: $[ 16\cos^4\theta-10\cos^2\theta+1]$ as this: $ \left( 2\cos^2 \theta-1 \right) \left( 8\cos^2 \theta-1 \right)$
So that it can be solved out for $\cos\theta$.
I know what the correct answer is but I'm trying to find out how to factorize similar cases. Did they use any trigonometric identity, or formula, that can help me?