f（x)=向量AB×向量BC=AB×BC×cos∠ABC=AB×BC×cos（2π/3）=AB×BC×-1/2BC/sinx=AC/sin∠ABC(正弦定理）BC=(sinx×AC)/sin∠ABC=2√3sinx/3因为∠ACB=π-2π/3-x=π/3-x同理AB/sin(π/3-x)=AC/sin∠ABCAB=（sin(...