s=COS(π/7)+COS(3π/7)+COS(5π/7)
2sinπ/7*s=2sinπ/7cosπ/7+2cos3π/7sinπ/7+2sinπ/7cos5π/7
=sin2π/7+(sin4π/7 -sin2π/7)+(sin6π/7-sin4π/7) (积化和差)
=sin6π/7
=sinπ/7
s=1/2
COS(π/7)+COS(3π/7)+COS(5π/7)化简求值,
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