\[\sin \left(\dfrac{2\pi}{7}\right)+\sin \left(\dfrac{4\pi}{7}\right)+\sin \left(\dfrac{8\pi}{7}\right) \]

If the value of the expression above is equal to \( \dfrac {\sqrt A}B \), where \(A\) and \(B\) are positive integers with \(A\) square-free, find \(A+B\).

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