cos[(a+b)/2]=cos([a-(b/2)]+[b-(a/2])=cos(a-(b/2))cos(b-(a/2))-sin(a-(b/2))sin(b-(a/2))由a,b的取值范围,sin(a-b/2)＞0cos(a/2-b)＞0∴sin(a-b/2)=√(1-(1/81))=4√5/9cos(a/2-b)=√(1-(4/9))=√5/3∴原式=-(√5...