cos(5π/8)*cosπ/8

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cos(5π/8)*cosπ/8cos(5π/8)*cosπ/8cos(5π/8)*cosπ/8cos(5π/8)*cos(π/8)=-sin(π/8)*cos(π/8)又∵sin(π/4)=2sin

cos(5π/8)*cosπ/8
cos(5π/8)*cosπ/8

cos(5π/8)*cosπ/8
cos(5π/8)*cos(π/8)=-sin(π/8)*cos(π/8)
又∵sin(π/4)=2sin(π/8)*cos(π/8)
∴cos(5π/8)*cos(π/8)=-sin(π/8)*cos(π/8)=-1/2(sin(π/4))
最后的答案应该是
负的四分之根号二