# Thread: Trig Identity

1. ## Trig Identity

So Im trying to prove the following inequality 8cos3θ6cosθ=1.
Ive tried a lot of different things, using substitutions for cos3θ, 2cosθ, pretty much everything and I cant seem to prove it.
Any help would be appreciated.
Thanks

2. ## Re: Trig Identity

That is neither an inequality nor an identity. WolframAlpha

Now, if you want to know for what values of $\theta$ you have $8\cos(3\theta) - 6\cos \theta = 1$, that is solvable, but not easy. You wind up with a cubic polynomial, which is solvable, but extremely difficult. Solution