由正弦定理：a/sinA=b/sinB=c/sinC=2R,
——》(a+b)/c=(sinA+sinB)/sinC=√3sinA+cosA,
——》sinA+sinB=√3sinAsinC+cosAsinC=sinA+sin(A+C)=sinA+sinAcosC+cosAsinC,
——》√3sinC-cosC=1=2sin(C-π/6),
——》sin(C-π/6)=1/2,
——》C=π/3.