Тригонометрия
sin 3Π/14 -sin Π/14 -sin 5Π/14=

Смотри решение во вложении

sin3π/14-(sinπ/14+sin5π/14)=sin3π/14-2sin3π/14cosπ/7=sin3π/14(1-2cosπ/7)=sin3π/14*2sin²π/14=2sinπ/14*1/2(cosπ/7-cos2π/7)=sinπ/14(cosπ/7-cos2π/7)