过A点作AE垂直BC于E,AB=根号3,∠B=30度,BE=ABcosB=根号3*(根号3/2)=3/2由于AE是等腰△ABC底边上的高,所以BE=CE,所以BC=2BE=3
BC²=AB²+AC²-2AB*AC*cos∠BAC =3+3-3*cos120° =6+3*1/2 =15/2BC=根号下15/2