AB，AC的数量积等于5即bccosA=5，又a=4，sin(B+C)=sinAtanA/tanB+tanA/tanC=sinA/cosA(cosB/sinB+cosC/sinC)=sinA/cosA[sin(B+C)/(sinBsinC)]=sin²A/(sinBsinCcosA)=a²/(bccosA)=16/5
