cosa=1/7,sina=4根号3/7
cos(a+b)=-11/14,sin(a+b)=5根号3/14
sin(b)
=sin(a+b-a)
=sin(a+b)cosa-sinacos(a+b)
=5根号3/14×1/7-4根号3/7×(-11/14)
=(5根号3+44根号3)/(7×14)
=根号3/2
b是锐角,所以b=派/3