Threatening Trigonometry

Geometry Level 4

$\cos^{ 4} \dfrac{\pi}{7} + \cos^{4} \dfrac{ 3 \pi}{7} + \cos^{4} \dfrac{5 \pi}{7}$

If the above expression is equals to $\dfrac {a}{b}$ where $a$ and $b$ are coprime positive.

Find $a+b$ .