a ,b都是锐角0≤a+b≤π ==>sina>0,sin(a+b)>0sina=√(1-cos²a)=4√3/7sin(a+b)=√[1-cos²(a+b)]=5√3/14cosb=cos[(a+b)-a]=cos(a+b)cosa+sin(a+b)sina=(-11/14)*(1/7)+(5√3/14)*(4√3/7)=1/2...