Gaokao Math Practice Question
在 $\triangle A B C$ 中, $B=\frac{\pi}{4}, B C$ 边上的高等于 $\frac{1}{3} B C$, 则 $\cos A$ 等于 ($\qquad$)
- A.$\frac{3 \sqrt{10}}{10}$
- B.$\frac{\sqrt{10}}{10}$
- C.$-\frac{\sqrt{10}}{10}$
- D.$-\frac{3 \sqrt{10}}{10}$
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