a b 都是锐角
COSa=1/7 ,sina=√(1-cos²a)=4√3/7
COS（a+b）=-11/14 ,sin(a+b)=√[1-cos²(a+b)]=5√3/14
COSb
=cos[(a+b)-a]
=cos(a+b)cosa+sin(a+b)sina
=(-11/14)*1/7+(5√3/14)*(4√3/7)
=1/2