高一数学题已知cosa
已知ca-cosb=1/2,sina-sinb=-1/3,求cos(a-b)的值
(cosa-cosb)=(1/2)=1/4,(sina-sinb)=(-1/3)=1/9, (cosa-cosb)+(sina-sinb)=2-2[cosa*cosb+sina*sinb]=2-2cos(a-b)=13/36,cos(a-b)=59/72.