xy=(2a+c)cosB+bcosC=0
2sinAcosB+sinCcosB+sinBcosC=0
2sinAcosB+sin(B+C)=0
2sinAcosB+sinA=0
sinA(2cosB+1)=0
而显然sinA不等于0,故2cosB=-1,B=120
a/sinA=c/sinC=(a+c)/(sinA+sinC)=b/sinB=2根6/3
a+c=2根6(sinA+sinC)/3
sinA+sinC=sinA+sin(A+B)
=sinA-0.5sinA+根3/2cosA
=1/2sinA+根3/2cosA
=sin(A+60)
而显然0