Gaokao Math Practice Question
9. (5 分) $\triangle A B C$ 的内角 A, B, C 的对边分别为 $a, b, C$. 若 $\triangle A B C$ 的面积为 $\frac{a^{2}+b^{2}-c^{2}}{4}$, 则 $\mathrm{C}=(\quad)$
- A.$\frac{\pi}{2}$
- B.$\frac{\pi}{3}$
- C.$\frac{\pi}{4}$
- D.$\frac{\pi}{6}$
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