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 $$a+b$$.

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