1,设a/sinA=b/sinB=c/sinC=k,
所以a=ksinA,b=ksinB,c=ksinC,因为bcosC=(3a-c)cosB,所以ksinBcosC=(3ksinA-ksinC)cosB
化简sinBcosC+sinCcosB=3sinAcosB,
sin(B+C)=3sinAcosB,sin(B+C)=sin(180-A)=sinA=3sinAcosB,所以cosB=1/3,sinB=(2根号2)/3
1,设a/sinA=b/sinB=c/sinC=k,
所以a=ksinA,b=ksinB,c=ksinC,因为bcosC=(3a-c)cosB,所以ksinBcosC=(3ksinA-ksinC)cosB
化简sinBcosC+sinCcosB=3sinAcosB,
sin(B+C)=3sinAcosB,sin(B+C)=sin(180-A)=sinA=3sinAcosB,所以cosB=1/3,sinB=(2根号2)/3