证明：
(a+b+c)²
=a²+b²+c²+2(ab+bc+ac)
=a²+b²+c²+2
=1/2(a²+b²)+1/2(a²+c²)+1/2(b²+c²)+2
≥ab+ac+bc+2
=1+2
=3,
仅当a=b=c=√3/3时,等号成立.