sina=4√3/7
cosa=1/7
cos(a+b)=-11/14
sin(a+b)=5√3/14
sinb=sin(a+b-a)=sin(a+b)cosa-cos(a+b)sina
=5√3/14*(1/7)+11/14*4√3/7
=(5√3+44√3)/98
=√3/2
b为锐角
b=60度