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题型：解答题 题类：其他 难易度：中档
已知\(0 < \alpha < \dfrac{\pi }{2},\sin \alpha =\dfrac{4}{5}\)
\((2)\)求\(\dfrac{\sin (\alpha +\pi )-2\cos (\dfrac{\pi }{2}+\alpha )}{-\sin (-\alpha )+\cos (\pi +\alpha )}\)的值．
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