cos 파/7*cos 2 파/7*cos 3/7 파

cos 파/7*cos 2 파/7*cos 3/7 파

원 식=sin pi/7*cos pi/7*cos 2 pi/7*cos 3 pi/7/sin pi/7
=2sinπ/7*cosπ/7*cos2π/7*cos(π-4π/7)/2sinπ/7
=sin2π/7*cos2π/7*(-cos4π/7)/2sinπ/7
=-2sin2π/7*cos2π/7*cos4π/7/4sinπ/7
=-sin4π/7*cos4π/7/4sinπ/7
=-2sin4π/7*cos4π/7/8sinπ/7
=-sin8π/7/8sinπ/7
=sinπ/7/8sinπ/7
=1/8