问题标题:
1,cos(π/7)+cos(3π/7)+cos(5π/7)1/2
问题描述:
1,cos(π/7)+cos(3π/7)+cos(5π/7)
1/2
刘明扬回答:
【用P表示圆周率】cosP/7+cos3P/7+cos5P/7=(2sinP/7*cosP/7+2sinP/7*cos3P/7+2sinP/7*cos5P/7)/(2sinP/7)=[sin2P/7+(sin4P/7-sin2P/7)+(sin6P/7-sin4P)]/(2sinP/7)=(sin6P/7)/(2sinP/7)=(sinP/7)/(2sinP/7)=1/2...
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