令AB=S，BC=S1，则AC=√3S1，∠ABC=120cos∠ABC=（S∧2+S1∧2-（√3S1）∧2）/（2S*S1）-1/2*2S*S1=（S∧2-2S1∧2）S∧2+S*S1-2S1∧2=0（S+2S1）（S-S1）=0S=S1，S=-2S1（舍）角度sin∠ABC/（√3S1）=sin∠ACB/（S1）sin∠ACB=1/2∠ACB=30°甲船沿着北偏东30°方向前进
