$\cos A$+$\cos B$+$\cos C$=$\cos A$+$2cos(B+C)/2$$\cos (B-C)/2$$\leq3/2$<=>$ 2\cos A+4\sin A/2.\cos (B-C)/2=\leq3$
<=>$2 -4sin^{2}A/2+4\sin A/2.\cos (B-C)/2\leq 3$
<=>$4\sin^{2}A-4\sin A/2.\cos (B-C)/2+1\geq 0$
<=>$(\sin A/2-\cos (B-C)/2)^{2}+\sin ^{2}(B-C)/2>=0$
cai nay luon dung roi. dau bang xay ra khi tam giac deu