f(B)=2sin(2B+2A+π/6)+1=0
sin(2B+2A+π/6)=-1/2=sin7π/6或sin11π/6
2A+2B+π/6=7π/6或11π/6
A+B=π/2或5π/6
C=π-(A+B)=π/2或π/6