Объяснение:
50+a=t
80+a=t+30
cost= sqrt(1-1/28)=-3sqrt(3)/2sqrt(7)
sin(t+30)=1/(2sqrt(7))*(sqrt(3)/2)-(1/2)*3sqrt(3)/2sqrt(7)=-2sqrt(3)/4sqrt(7)=
=-(1/2)*sqrt(3/7)
sqrt(7/3)*sin(80+a)=-1/2
