Ясно, что ВН = 8 - 2*(4 - √7) = 2*√7; cos(B)=BH/BA = √7/4;
sin(B) = √(1 - (cos(B))^2) = √(1 - 7/16) = √(9/16) = 3/4;
2*sin(HBC)=sin(ABC)=2*HC/BC=4*(4-sqrt(7))/8=2-sqrt(7)/2
