Question

sina-sinp=-1/2,cosa-cosp=1/3,则cos（a-p）=?

Answer 1

(sina-sinp)^2=(sina)^2+(sinp)^2-2sinasinp=1/4 sinasinp=3/8 (1)

(cosa-cosp)^2=(cosa)^2+(cosp)^2-2cosacosp=1/9 cosacosp=4/9 (2)

(1)+(2)：cos(a-p)=59/72