已知sin(π+α)=-1/2 计算 cos(2π-α) tan(α-7π)

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解答：
因咐模念码卖为：衡困sin(π+α)=-1/2 ，所以：sinα=1/2
cos(2π-α) tan(α-7π) =cosα*tanα=sinα=1/2

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