设单位圆上禅则有两点M(cosA,sinA),N(cosB,sinB) 则向量OM=(cosa,sina),ON=(cosb,sinb) |OM|=|ON|贺迅棚昌裂=1 OM·ON=|OM|*|ON|cos(a-b)=cosacosb+sinasinb 故:cos(a-b)=cosacosb+sinasinb