Gaokao Math Practice Question
若函数 $f(x)$ 的定义域为 $\mathbf{R}$, 且 $f(x+y)+f(x-y)=f(x) f(y), f(1)=1$, 则 $\sum_{k=1}^{22} f(k)=(\quad)$
- A.$-3$
- B.$-2$
- C.0
- D.1
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