cos(a+b)*cos(a-b)
=(cosacosb-sinasinb )*(cosacosb+sinasinb)
=(cosacosb)^2 -(sinasinb)^2
(cosa)^2 -(cosb)^2
=(cos2a - cos2b)/2
=-sin(a+b)sin(a-b)
原题有误吧