Let $ABC$ be a triangle with side lengths $AB=6$, $AC=7$, and $BC=8$. Let $H$ be the orthocenter of $\triangle ABC$ and $H'$ be the reflection of $H$ across the midpoint $M$ of $BC$. The expression $\frac{[ABH']}{[ACH']}$ can be written as $\frac{p}{q}$. Find $p+q$.

Let's think step by step and output the final answer within \boxed{}.
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