∵0＜b＜a＜π/2 ==>00 ∵cosa=1/7,cos(a-b)=13/14 ∴sina=√[1-(cosa)^2]=4√3/7 sin(a-b)=√[1-(cos(a-b))^2]=3√3/14 则cosb=cos[a-(a-b)]=cosacos(a...