cosπ/5·cos2π/5等于多少?

cosπ/5·cos2π/5等于多少?
cosπ/5·cos2π/5等于多少?

cosπ/5·cos2π/5等于多少?
cosπ/5*cos2π/5
=(2sinπ/5*cosπ/5*cos2π/5)/(2sinπ/5)
=(sin2π/5*cos2π/5)/(2sinπ/5)
=(2sin2π/5*cos2π/5)/2*(2sinπ/5)
=sin4π/5/(4sinπ/5)
=sin(π-π/5)/(4sinπ/5)
=sin(π/5)/(4sinπ/5)
=1/4

1/4