解：cos(a+b)=-5/13,=> sin(a+b)=12/13,=> tan(a+b)=-12/5,tana=3/4,=> tan(a+b)=(tana+tanb)/(1-tana*tanb)=(3/4+tanb)(1-3tanb/4)=-12/5,=> tanb=63/16,=> cosb=16/65
