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若 $\alpha \in\left(\frac{\pi}{4}, \frac{3 \pi}{4}\right), 6 \tan \left(\frac{\pi}{4}+\alpha\right)+4 \cos \left(\frac{\pi}{4}-\alpha\right)=5 \cos 2 \alpha$, 则 $\sin 2 \alpha=$
A. $\frac{24}{25}$     B. $\frac{12}{25}$     C. $\frac{7}{25}$     D. $\frac{1}{5}$         
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