Question

cosπ/5·cos2π/5等于多少?

Answer 1

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