由cosa = 1/7 知 sina = 4√3/7.cos(a-b) = cosa*cosb+sina*sinb∴1/7·cosb + 4√3/7·sinb = 13/14.-①∵cos²b + sin²b = 1.-②联立①②得196cos²b - 52cosb - 23 = 0.解之,得cosb = 1/2 或 -23/98...