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
