cos5π/8*cosπ/8
=cos(4π/8+π/8)*cosπ/8
=(cosπ/2*cosπ/8-sinπ/2*sinπ/8)*cosπ/8
=-sinπ/8*cosπ/8
=-1/2*2sinπ/8*cosπ/8
=-1/2*sinπ/4
=-根号2/4