I've been practicing trigonometric equations lately and I've stumbled upon this problem
$$\cos x + \cos3x = 2$$
I tried to write $\cos3x$ as $\cos(2x+x)$ and then I got $\cos2x \cdot \cos x - \sin2x\sin x$
Then when I finally make the equations as simple like this:
$$\cos^2x(\cos x - 1) = 2$$
I don't know what to do anymore. Any help would really appreciate it.