cosa=5/13,cos(a+b)=-4/5,求cosb=求出sina=12/13.sina(a+b)=-3/5cos(a+b)=cosacosb-sinasinb=-4/5代入缺少两个条件:sinb,cosb怎么求

cosa=5/13,cos(a+b)=-4/5,求cosb=
求出sina=12/13.
sina(a+b)=-3/5
cos(a+b)=cosacosb-sinasinb=-4/5
代入缺少两个条件:sinb,cosb
怎么求

cosb=cos(a+b-a)
=cos(a+b)*cosa+sin(a+b)*sina
=5/13*(-4/5)+(-3/5)*12/13
=56/65