c2=a2+b2-2abcosC 2√3absinC=a2+b2+c2 则,2√3absinC+2abcosC=2(a2+b2) 即√3absinC+abcosC=a2+b2 得2sinC+30)=a2+b2 所以(a-b)^2=2ab[sin(C+30)-1] 因为(a-b)^2>=0,sin(C+30)-1