cosa+cosb=1/3 ,sina+sinb=1/2分别两边平方(cosa)^2+2cosacosb+(cosb)^2=1/9(sina)^2+2sinasinb+(sinb)^2=1/4相加且(cosa)^2+(sina)^2=1 (cosb)^2+(sinb)^2=12+2(cosacosb+sinasinb)=1/4+1/9=13/36cos(a-b)=cosacosb...