因為a^2=b^2+c^2-2bccosA
S=(1/2)bcsinA
則a^2+b^2+c^2-4√3S
=b^2+c^2-2bccosA+b^2+c^2-4√3*(1/2)bcsinA
=2b^2+2c^2-2bccosA-2√3bcsinA
=2b^2+2c^2-4bc[(1/2)cosA+(√3/2)sinA]
=2b^2+2c^2-4bc+4bc-4bccos(60-A)
=2(b-c)^2+4bc[1-cos(60-A)]
-120 < 60-A < 60
-1/2 < cos(60-A) ≤ 1
0 ≤ 1-cos(60-A) < 3/2
所以
a^2+b^2+c^2-4√3S = 2(b-c)^2+4bc[1-cos(60-A)] ≥0
當b=c且A=60時,即等邊三角形時,等號成立.