Gaokao Math Practice Question
10. （5 分) 若 $f(x)=\cos x-\sin x$ 在 $[-a, a]$ 是减函数, 则 $a$ 的最大值是 $(\quad)$
- A.$\frac{\pi}{4}$
- B.$\frac{\pi}{2}$
- C.$\frac{3 \pi}{4}$
- D.$\pi$
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