a为锐角,则sina=4√3/7,sin(a+B)=5√3/14sinB=sin[(a+B)-a]=sin(a+B)cosa-cos(a+B)sina=5√3/14*1/7-(-11/14*4√3/7)=√3/2cos(2a+B)=cos[(a+B)+a]=cos(a+B)cosa-sin(a+B)sina=-11/14*1/7-(5√3/14*4√3/7)=-71/98...