ab都是锐角,cosa=1/7,cos(a+b)=11/14
则sina=√1-cos²a=4√3/7
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
=11/98+60/98
=71/98