It is known that cos (a-b / 2) = - 1 / 9, sin (a-2-b) = 2 / 3, and 0

It is known that cos (a-b / 2) = - 1 / 9, sin (a-2-b) = 2 / 3, and 0

∵cos(a-b/2)=-1/9,
0