sinπ/10*cosπ/5
=（cosπ/10*sinπ/10*cosπ/5）/（cosπ/10）
=（1/2sinπ/5cosπ/5）/（cosπ/10）
=（1/4sin2π/5）/sin（π/2-π/10）
=（1/4sin2π/5）/sin2π/5
=1/4