Sin4a-sin5a-sin6a+sin7a=-4sina/2*sina*sin11a/2

sin4a+sin7a — (sin5a+sin6a)=-4sina/2*sina*sin11a/22sin (11a/2) cos (3a/2) — 2sin (11a/2) cos (a/2)=-4sina/2*sina*sin11a/22sin (11a/2)*(cos (3a/2) -cos (a/2)+2*sina/2*sina)=02sin (11a/2)*(cos (3a/2) -cos (a/2)+cos (a/2) -cos (3a/2)=0sin (11a/2)=0 11a/2=pi*ka=2/11*pi*k где k- целые числа