Gaokao Math Practice Question
已知 $\alpha$ 为第二象限角, $\sin \alpha+\cos \alpha=\frac{\sqrt{3}}{3}$, 则 $\cos 2 \alpha=(\qquad)$
- A.$-\frac{\sqrt{5}}{3}$
- B.$-\frac{\sqrt{5}}{9}$
- C.$\frac{\sqrt{5}}{9}$
- D.$\frac{\sqrt{5}}{3}$
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