sin（2π/7）+sin（3π/7）-sin（π/7）=sin(5π/14-π/14)+sin(5π/14+π/14)-sinπ/7=sin5π/14cosπ/14-cos5π/14sinπ/14+sin5π/14cosπ/14+cos5π/14sinπ/14-sinπ/7=2sin5π/14cosπ/14-sinπ/7=2cos(2π/14...