2)f(2π/3-a/2)=3sin(π-a)=9/5sina=3/5f(b/2+5π/12)=3sin(π/2+b)=-36/13cosb=-12/13所以,sina=3/5 ,cosa=4/5sinb=5/13 ,cosa=-12/13cos(a+b)=cosacosb-sinasinb=-4/5*12/13-3/5*5/13=-63/65cos(a+b)=-63/65
