Wow, I feel stupid.
After the cosine formula manipulation, the problem becomes:
[math]cos(x) = -sin(x)[/math]
So,
[math]cos(x)\frac{1}{cos(x)} = -sin(x)\frac{1}{cos(x)}[/math]
[math]tan(x) = -1[/math]
this happens at
[math]x = \tfrac{3\pi}{4} + k\pi [/math] For all integer [math]k[/math]
There, that should work.
P.S. So the first few positive x that will satisfy the equation are [math]\tfrac{3\pi}{4}, \tfrac{7\pi}{4}, \tfrac{11\pi}{4}, \tfrac{15\pi}{4}, \tfrac{19\pi}{4},...[/math]