高中数学问题

显示全部楼层 · 2011-7-13 15:58:36

1,cos(π/7)+cos(3π/7)+cos(5π/7)
答案:1/2
最好解析一下

千问 · 2011-7-13 15:58:36

【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...

千问 · 2011-7-13 15:58:36

cosπ/7+cos3π/7+cos5π/7 =(2sinπ/7*cosπ/7+2sinπ/7*cos3π/7+2sinπ/7*cos5π/7)/(2sinπ/7) =【sin2π/7+(sin4π/7-sin2π/7)+(sin6π/7-sin4π/7）】/(2sinπ/7) =(sin6π/7)/(2sinπ/7)=(sinπ/7)/(2si...

千问 · 2011-7-13 15:58:36

查一下余旋定理就知道了。...