sina=4√3/7,sin(a+b)=5√3/14
cos(a+b)=cosacosb-sinasinb=-11/14
sin(a+b)=sinacosb+cosasinb=5√3/14
即：
1/7*cosb-4√3/7*sinb=-11/14
4√3/7*cosb+1/7*sinb=5√3/14
即：cosb=1/2
b属于(0,π/2),
b=π/3